Age at menopause:
Associated factors and temporal trends
Marthe Sørli Gottschalk
Department of Obstetrics and Gynecology Akershus University Hospital
and
Institute of Clinical Medicine University of Oslo
Campus Akershus University Hospital
PhD Thesis Faculty of Medicine
University of Oslo 2021
© Marthe Sørli Gottschalk, 2022 Series of dissertations submitted to the Faculty of Medicine, University of Oslo
ISBN 978-82-8377-979-0
All rights reserved. No part of this publication may be
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Cover: Hanne Baadsgaard Utigard.
Print production: Reprosentralen, University of Oslo.
Table of content
Acknowledgements ... 4
Abbreviations ... 6
List of papers ... 8
1 Introduction ... 9
2 Background ... 10
2.1 The definition of menopause ... 10
2.2 The variation in age at menopause ... 11
2.2.1 Inter-individual variation ... 11
2.2.2 Geographical variation ... 16
2.2.3 Variation across birth cohorts ... 17
2.3 The underlying biological mechanisms of menopause ... 18
3 Background for the present studies ... 26
3.1 Temporal trends in age at menarche and menopause ... 26
3.2 Number of childbirths and age at menopause ... 28
3.3 Childbirth close to natural menopause ... 29
4 Objectives for the thesis ... 30
5 Material and methods ... 31
5.1 The BreastScreen Norway ... 31
5.1.1 Paper I – Study sample, variables and statistical analyses ... 32
5.1.2 Paper II – Study sample, variables and statistical analyses ... 35
5.2 The HUNT2 Survey ... 38
5.2.1 Paper III – Study sample, variables and statistical analyses ... 39
5.3 Ethical aspects ... 43
6 Synopsis of the studies ... 44
7 Discussion ... 47
7.1 Main findings ... 47
7.2 Methodologic considerations ... 47
7.3 Interpretation of results ... 54
8 Conclusions, clinical implications and future perspectives ... 58
9 References ... 60
Appendix ... 70
Acknowledgements
I count myself very fortunate to have Elisabeth K. Bjelland and Anne Eskild as my
supervisors and mentors, who have renounced their spare time, weekends and holidays to give me feedback. I would like to express my deepest gratitude to my inspiring, and always
enthusiastic main supervisor, Elisabeth. You took me deeper into the fascinating world of research after I completed my master degree in Public Health. You have thought me complex statistical methods and advanced epidemiological thinking. Thank you for all the intense, but professional conversations, and to always keep an eye for structure and details. I also
appreciate your motivational words when needed. I will always be grateful for the patience you have had with me.
I also owe a particular gratitude to my second supervisor Professor Anne Eskild. It has been a great privilege to have you as a supervisor with such extensive knowledge and experience in female health research and in the field of epidemiology. You thought me critical thinking and gave concrete answers to all my questions. Thank you for following me all the way, and for all the constructive feedback you have given me since the beginning.
A special thanks to Solveig Hofvind who was responsible for the data collection of the
women participating in BreastScreen Norway. This data has given me a unique opportunity to study a large part of the Norwegian female population. I also want to thank Jon Michael Gran and Tom Tandbo for valuable contributions to our papers.
I am very thankful to my research fellows at the Department of Obstetrics and Gynecology. I truly appreciate the sharing of successes and frustrations, academic conversations and
motivational talks. I have also appreciated the opportunity of presenting our research to the staff at the Department. Thank you for asking questions and for all your comments, it has contributed to better precision in our studies.
To my dear friends, thank you for your endless support and for always being there for me.
My dearest Evyatar, thank you for all the comforting hugs, motivational words and for always believing in me. I love you.
Finally, to my sweetest Siri. You are the best thing that have ever happened to me, and your smile and kisses every day have increased my motivation to complete this thesis. Soon you will become a big sister, and I am sure you will be a great inspiration and motivation for her as well.
Lørenskog, May 2021 Marthe S. Gottschalk
Abbreviations
CI Confidence interval
FSH Follicle-stimulating hormone
GnRH Gonadotropin-releasing hormone
HR Hazard ratio
LH Luteinizing hormone
List of papers
I. Temporal trends in age at menarche and age at natural menopause: a population study of 312 656 women in Norway. M.S. Gottschalk, A. Eskild, S. Hofvind, J.M. Granand E.K. Bjelland. Human Reproduction. 2020;35:464-471.
II. The relation of number of childbirths with age at natural menopause: a population study of 310 147 women in Norway. M.S. Gottschalk, A. Eskild, S. Hofvind,and E.K.
Bjelland. Submitted to journal in March 2021, and in revision at the time of PhD thesis submission.
III. Childbirth close to natural menopause: does age at menopause matter? M.S.
Gottschalk, A. Eskild,T.G. Tanbo, and E.K. Bjelland. Reproductive BioMedicine Online. 2019;39:169-175.
1 Introduction
Menopause is defined as the permanent cessation of menstrual periods, and represents the end of a woman’s reproductive life (1). Women’s age at natural menopause varies considerably between women, and the underlying mechanisms that cause the large variation are not well understood (2). Age at natural menopause also displays great geographical variation (3), and may vary across birth cohorts (4). The timing of menopause influences the length of the period when reproduction is possible, and also the risk of disease and early death (5-9). Thus, valid knowledge about temporal trends in age at natural menopause may be important for the understanding of population changes in fertility at high reproductive ageand the disease burden after menopause.
Identification of factors that are associated with age at menopause, may improve our understanding of the mechanisms that causes the variation in age at menopause. Although genetic factors play an important role (10), endogenous and exogenous factors that may alter ovarian functions have been associated with age at menopause (11). It is well known that smokers reach menopause 1-2 years earlier than non-smokers (12, 13). High body mass index and oral contraceptive use have been associated with high age at menopause. However, previous findings are not consistent (14-17). Nulliparous women reach menopause earlier than parous women (14, 18). Whether or not age at menopause increases with number of childbirths is uncertain (11, 15).
The time interval between menarche and menopause roughly defines a woman’s period for reproduction, and women’s fecundity at the end of this period is assumed to be low (19).
During the past decades, women’s mean age at first childbirth has increased worldwide (20, 21). If mean age at menopause has remained stable, the women’s number of years for
reproduction may be reduced. Yet, the chance of having a successful spontaneous pregnancy in the years close to menopause is not known.
Thus, the aims of this thesis were to study if age at natural menopause has changed over time, to study if age at menopause increases by number of childbirths, and to study whether a successful spontaneous pregnancy is possible in the years close to menopause.
2 Background
2.1 The definition of menopause
Menopause occurs when the number of ovarian follicles reaches a critical low level. Natural menopause occurs spontaneously and is caused by loss of ovarian follicles (22, 23). Age at natural menopause can be reported with certainty one year after the last menstrual period (24).
Perimenopause is defined as the time period immediately prior to the last menstrual period and the first year after (25) (Figure 2-1), and includes the early and late menopausal
transitions. The early menopausal transition is defined as loss of regular menstrual cycles or missed menstrual cycles, but the cycles that occur are usually ovulatory. The late menopausal transition is characterized by shorter and anovulatory cycles (3).
Early menopause is defined by cessation of menstrual periods before the age of 45, and occurs in 3-5% of all women (26-29). Primary ovarian insufficiency is defined by cessation of menstrual periods before the age of 40, and occurs in 0.3-1.2% of all women (26, 27, 29, 30). Some women may reach menopause because of medical treatment that affects ovarian functions, such as chemotherapy or radiotherapy, or after surgical removal of both ovaries (3).
Postmenopause is defined as the time period dating from the last menstrual period, regardless of whether the menopause occurs spontaneously or is induced (25) (Figure 2-1).
Today, most women reach menopause long before they die and more than one third of a woman’s life remains after the menopause (31). Why female human reproduction ends long before life itself is an unanswered evolutionary question (32-35). Although the menopause is an event that occurs long before death in most women, both early and late menopause have been associated with adverse health outcomes after menopause (5-7, 36-41).
Figure 2-1. The stages of reproductive ageing in women (3). Figure created by using SmartArt in Microsoft Word 2013.
2.2 The variation in age at menopause
In a recent meta-analysis of 36 studies across 35 countries, the estimated mean age at natural menopause was 48.8 years (42). However, there is a large variation between women in age at menopause. Almost all women reach natural menopause between the age of 40 and 60 years (4, 42).
2.2.1 Inter-individual variation
The inter-individual variation in age at menopause is large. Genetic factors play an important role for the timing of menopause (43-45), and several studies have found a strong association between a mother’s and her daughter’s age at natural menopause (10, 46-49). However, genetic factors alone cannot explain the large variation. Several other factors have also been associated with age at menopause, and a selection of such known factors are presented below (Table 2-1).
Lifestyle factors
Women who smoke cigarettes daily are likely to reach menopause 1-2 years earlier than non- smokers (12, 13, 50, 51). Studies have suggested that tobacco smoke contains compounds that may be toxic to the ovarian follicles, resulting in premature loss of ovarian follicles (52, 53) and thereby earlier menopause (54). Overweight and obesity have been associated with high age at menopause (55). Estrogen is synthesized in fatty tissue and increased peripheral production of estrogens is hypothesized as a contributor to delay menopause. However, the association between body mass index with age at menopause is inconsistent (4, 14, 15, 17, 56- 62). Also the association of physical activity and alcohol consumption with age at menopause
is uncertain (15, 51, 60-65). However, women with high socioeconomic status have consistently been reported to have higher age at menopause, compared to women with low socioeconomic status (14, 15, 56, 63, 66).
Early life factors
Factors in early life, such as birthweight and childhood weight, have been suggested to influence age at menopause (46, 67-74). Low birthweight is often used as a proximate for adverse environment in fetal life (75). Thus, low birthweight could be an indicator for poor development of the ovaries and ovarian follicles, and thereby earlier age at menopause. The association between birthweight and age at menopause have been investigated previously, but with inconsistent results (46, 67-69, 71, 76). Some studies report that high weight at age one and two years has been associated with increasing age at menopause (46, 67, 69). While another study found no association between weight at age 15 and age at menopause (17).
Also, poor weight gain during early childhood has been associated with earlier menopause (69, 74).
Reproductive factors
Factors that may reduce or inhibit ovulation, such as prolonged breastfeeding, long menstrual cycle length, oral contraceptive use, and high number of pregnancies have been suggested to be associated with late menopause (14, 18, 60, 63, 65). It is not established whether oral contraceptive use has any effect on the timing of menopause. Some studies have shown that the use of oral contraceptives is associated with late menopause (14, 15, 77-80), while other studies report no association (16, 81-83). Also number of childbirths has been associated with age at menopause, and it is well known that nulliparous women reach menopause earlier than parous women (14, 18). However, whether age at menopause increases with number of childbirths has been insufficiently studied (11, 15). Age at menarche may determine the number of menstrual cycles during the life course, and each menstrual cycle recruits several ovarian follicles that degenerate. Age at menarche has therefore been suggested to be associated with age at menopause, and many studies have assessed the relation of age at menarche with age at menopause (84). Some studies suggest that women with early menarche also have early menopause (56, 73, 85-89), or that women with late menarche have late menopause (4, 62). Few studies report that women with early menarche have late menopause (80, 90), but most studies suggest that no association exist (81, 82, 91-96).
Table 2-1. The associations of lifestyle factors, early life factors, and reproductive factors with age at menopause.*
Age at menopause
Increased Decreased No association
Lifestyle factors
Cigarette smoking Dratva et al. 2009 (4),
Sun et al. 2012 (12), Harlow and Signorello 2000 (13), Gold et al.
2001 (14), Mikkelsen et al. 2007 (50), Gold 2011 (51), Mattison and Thorgeirsson 1978 (54), Kaczmarek 2007 (56), Willet et al. 1983 (57), Nagata et al. 1998 (59), Bromberger et al. 1997 (60), Emaus et al. 2013 (62), Celentano et al.
2003 (63), Kinney et al.
2006 (64), Nilsson et al.
1997 (97), Dorjgochoo et al. 2008 (98)
Low body mass index Dratva et al. 2009 (4),
Gold et al. 2013 (15), Akahoshi et al. 2002 (55), Willet et al. 1983 (57), Nagata et al. 1998 (59), Morria et al. 2012 (61), Emaus et al. 2013 (62), Dorjgochoo et al.
2008 (98)
Gold et al. 2001 (14), Hardy et al. 2008 (17), Kaczmarek 2007 (56), Bromberger et al. 1997 (60)
High body mass index Gold et al. 2013 (15), Akahoshi et al. 2002 (55), Nagata et al. 1998 (59), Morris et al. 2012 (61), Emaus et al. 2013 (62), Dorjgochoo et al.
2008 (98)
Dratva et al. 2009 (4) Gold et al. 2013 (14), Hardy et al. 2008 (17), Kaczmarek 2007 (56), Bromberger et al. 1997 (60)
Low physical activity Dratva et al. 2009 (4),
Gold et al. 2013 (15)
Gold et al. 2013 (14), Hardy et al. 2008 (17), Kaczmarek 2007 (56), Bromberger et al. 1997 (60)
High physical activity Morria et al. 2012 (61), Emaus et al. 2013 (62), Dorjgochoo et al. 2008 (98)
Gold et al. 2001 (14), Hardy et al. 2008 (17), Kaczmarek 2007 (56), Bromberger et al. 1997 (60)
Alcohol consumption Gold et al. 2013 (15), Morris et al. 2012 (61), Kinney et al. 2006 (64)
Kaczmarek 2007 (56), Celentano et al. 2003 (63)
Low socioeconomic status
Gold et al. 2001 (14), Gold et al. 2013 (15), Kaczmarek 2007 (56), Celentano et al. 2003 (63), Luoto et al. 1994 (66)
High socioeconomic status
Gold et al. 2001 (14), Gold et al. 2013 (15), Kaczmarek 2007 (56), Celentano et al. 2003 (63), Luoto et al. 1994 (66)
Early life factors
Low birthweight Steiner et al. 2010 (71),
Tom et al. 2010 (72), Ruth et al. 2016 (73)
Mishra et al. 2007 (46), Cresswel et al.1997 (67), Treloar et al. 2000 (68), Hardy and Kuh 2002 (69)
High birthweight Tom et al. 2010 (72) Mishra et al. 2007 (46),
Cresswel et al.1997 (67), Treloar et al. 2000 (68), Hardy and Kuh 2002 (69)
Low childhood weight Mishra et al. 2007 (46),
Cresswel et al.1997 (67), Hardy and Kuh 2002 (69)
Hardy et al. 2008 (17)
High childhood weight Mishra et al. 2007 (46), Cresswel et al.1997 (67), Hardy and Kuh 2002 (69)
Hardy et al. 2008 (17)
Reproductive factors
Early menarche Kaczmarek 2007 (56), Nagata et al. 1998 (59), Hardy and Kuh 2002 (69),
Ruth et al. 2016 (73), Henderson et al. 2008 (87), Brand et al. 2015 (88), Li et al. 2016 (89)
Van Keep et al. 1979 (80), Boulet et al. 1994 (90)
Nagel et al. 2005 (81), Van Noord et al. 1997 (82), Bjelland et al.
2014 (91), Kato et al.
1998 (92), Yasui et al 2012 (93), Zsakai et a.
2015 (94), Rizvanovic et al. 2013 (95), Otero et al. 2010 (96) Late menarche Dratva et al. 2009 (4),
Emaus et al. 2013 (62)
Nagel et al. 2005 (81), Van Noord et al. 1997 (82), Bjelland et al.
2014 (91), Kato et al.
1998 (92), Yasui et al 2012 (93), Zsakai et a.
2015 (94)
Nulliparity Dratva et al. 2009 (4),
Gold et al. 2001 (14),
Gold et al. 2013
Cramer et al. 1995 (18), Mishra et al. 2007 (46), Kaczmarek 2007 (56), Nagata et al. 1998 (59), Morris 2012 (61), Celentano et al. 2003 (63), Standford et al.
1987 (83), Henderson et al. 2008 (87),
Dorjgochoo et al. 2008 (98), Li et al. 2012 (99) Parity Dratva et al. 2009 (4),
Gold et al. 2001 (14), Cramer et al. 1995 (18), Mishra et al. 2007 (46), Kaczmarek 2007 (56), Nagata et al. 1998 (59), Morris 2012 (61), Celentano et al. 2003 (63), Standford et al.
1987 (83), Henderson et al. 2008 (87),
Dorjgochoo et al. 2008 (98), Li et al. 2012 (99)
Gold et al. 2013
Oral contraceptive use Gold et al. 2001 (14), Gold et al. 2013 (15), Palmer et al. 2003 (77), Torgerson et al 1994 (78), Gonzales and Villena 1997 (79), Van Keep et al. 1979 (80)
De Vries et al. 2001 (16), Nagel et al.2005 (81),Van Noord 1997 (82),
Standford et al. 1987 (83)
Breastfeeding Cassou et al. 1997 (28), Mishra et al. 2007 (46), Dorjgochoo et al. 2008 (98), Chang et al. 2007 (101)
Kaczmarek 2007 (56)
Short menstrual cycles Cramer et al. 1995 (18), Bromberger et al. 1997 (60), Celentano et al.
2003 (63), Dorjgochoo et al. 2008 (98)
Long menstrual cycles Cramer et al. 1995 (18), Bromberger et al. 1997 (60), Celentano et al.
2003 (63), Dorjgochoo et al. 2008 (98)
*References are presented in numbers in parenthesis and correspond to the numbers in references list at the end of this thesis.
2.2.2 Geographical variation
Mean age at natural menopause varies between 44.6 and 54.5 years according to geographic regions (4, 42, 97) (Figure 2-2). Some studies have suggested that women who live in urban areas have natural menopause later than women do in rural areas (51, 98). Women who live in developed countries seem to reach menopause later than women who live in developing countries (79, 99-103). Thus, women from Africa, Middle East, Latin America and Asia reach menopause earlier than women from Europe, Australia, Canada, and the USA (42). The geographic variation in age at menopause may be a result of differences in genetic, ethnical, socioeconomic, environmental, and/or lifestyle factors (51). Also, in both developed and developing countries, women’s lifestyles have changed during the previous decades (104- 106). Whether age at menopause is altered in conjunction with these changes remains uncertain (4, 107).
According to the map below (Figure 2-2), the mean age at menopause among women in Norway is reported to be 48.4 years (5). This estimate was based on women born during the years 1887-1929 living in areas of Vestfold, Nord-Trøndelag and Aust-Agder. The data was collected in 1961, and the areas were considered rural at data collection. Thus, mean age at menopause may not be representative for the actual mean age at menopause of the entire Norwegian female population. Further, the neighbor countries, Norway and Sweden, have similar population characteristics, thus it may be questioned why the estimated mean age at menopause was 2.4 years higher in Sweden than in Norway. The estimated mean age at menopause in Sweden was based on women born in Gothenburg, the second largest city in Sweden, and they were born during the years 1915-1941. Thus, the differences in birth year and living area could have contributed to the substantial difference in the mean age at menopause between Norway and Sweden reported in the map.
Figure 2-2. The variations in mean age at menopause across countries. Adapted from Laisk T, Tšuiko O, Jatsenko T, Hõrak P, Otala M, Lahdenperä M, et al. Demographic and evolutionary trends in ovarian function and aging. Hum Reprod Update. 2019;25(1):34-50 (108).
2.2.3 Variation across birth cohorts
A birth cohort is defined as a group of individuals born in the same year (109), and age at menopause may vary across birth cohorts. However, knowledge regarding a possible temporal trend in age at menopause is limited (4, 91, 107, 110-112). Yet, a variation in age at
menopause across birth cohorts has been reported (4, 91, 107, 110-112). A population study of women from North-Trøndelag county in Norway reported that mean age at menopause was 50.6 years and estimated a decrease in age at menopause across birth cohorts from 1925 to 1977 (91). Also, some other population studies from Europe and USA, suggest an increase in age at menopause across birth cohorts among women born in the beginning of 1900 until the 1950s (4, 110-112). In a study of Portuguese women age at menopause increased across the birth cohorts from 1900 to 1932, and a decrease in age at menopause was observed after the 1932 birth cohort until the 1963 birth cohort (107). Most of the above studies were based on small non-representative samples, did not include recent birth cohorts, or had methodical
limitations. Thus, large population studies of temporal trends in age at menopause in recent generations are warranted.
Epidemiological studies have suggested that prenatal and early postnatal environment may influence adult health (113-115). During the 19th and first part of the 20th century, people endured battles and revolutions, world wars and the Great Depression. People born in the last part of the 20th century were born into an advanced technological society (70, 116-118).
Consequently, economical, medical, environmental, nutritional and life style changes over time may have influenced the mean age at natural menopause, as these factors have been associated with the timing of menopause (70, 119).
2.3 The underlying biological mechanisms of menopause
Menopause is assumed to occur when there is a limited number of oocytes left in the ovaries as a result of oocyte degeneration throughout the woman’s reproductive life (120-123). The low oocyte count in combination with age-related decline in quality of the oocytes are important factors for the low fecundity in aging women (124).
2.3.1 The quantity and quality of oocytes
The oocytes are female sex cells derived from primordial germ cells (125). During 11-12 weeks of fetal life, the primordial germ cells arrive in the ovaries and differentiate into oogonia. Some of the oogonia are arrested in the first meiotic division (meiosis I) and form primary oocytes. The number of oogonia and primary oocytes increase rapidly in the ovary and peak with a total of 6-7 million at 20 weeks of fetal life (122). At this time, cell death begins, and many oogonia and primary oocytes undergo degeneration (atresia) (125). At birth,
1-2 million oocytes remain in the ovaries (120-123). From birth to puberty the number of oocytes continue to decline, and at menarche, 300-400 000 oocytes remain in the ovaries.
During the reproductive life, the atresia of oocytes continues, and menopause has been estimated to occur when less than 1000 oocytes remain in the ovaries (120-123).
Meiosis
Meiosis is the process in which the sex cells divides twice to produce four cells containing half of the original amount of genetic information (125) (Figure 2-3). Meiosis is divided into meiosis I and II, and both have the same arrangement of phases called prophase, metaphase,
anaphase and telophase. Both meiosis I and II produce two daughter cells from each parent cell. However, meiosis I begin with one diploid parent cell and ends with two haploid daughter cells, whereas in meiosis II four haploid daughter cells are produced (126). Female meiosis is a long process, in which the meiosis of the oocytes is arrested twice. The first arrest occurs in prophase of meiosis I, and here they are arrested until the oocytes are ovulated (125). Less than 500 oocytes will be ovulated throughout a woman’s reproductive life (122).
Thus, some oocytes that reach maturity late in life have possibly been arrested in prophase of meiosis I for 40 years or more before ovulation (126). The second arrest lasts for only a few hours and occurs in metaphase of meiosis II. Meiosis II is completed upon fertilization (125).
As a woman ages, the quality of the oocytes decreases, and a defective process of meiosis is assumed to cause the low quality (127, 128). Errors in the spindle formation and chromosome alignment during meiosis are believed to cause failures in ovulation and/or fertilization. If fertilization is completed in aging women, it may cause oocyte embryo aneuploidy, which is the most common cause of miscarriages in aging women (129). Also, instability in the mitochondrial DNA of the oocyte is assumed to play an important role (130-132).
Figure 2-3. Meiosis (125). Figure created by using https://smart.servier.com/smart_image/embryology/
2.3.2 Development and degeneration of the ovarian follicles
The ovarian follicles serve as the functional units of the ovaries and consist of an oocyte surrounded by granulosa cells (133-135). The ovarian follicles develop into primordial, primary, secondary and antral follicles, whereas only a few of these develop into the final Graafian follicle (136) (Figure 2-4). As the primary oocyte begins to grow, the surrounding follicular cells change from flat epithelial cells into stratified epithelial cells, also called granulosa cells, and form primary follicles (133-135) (Figure 2-4). The granulosa cells start to get surrounded by theca cells. As the follicles grow, small finger-like processes of follicular cells extend across the zona pellucida and connect with microvilli of the plasma membrane of the oocyte. This is important for the transport of materials from follicular cells to the oocyte.
Further, in the follicular development, a fluid-filled space called antrum is formed and secondary follicles and antral follicles are created (125). At the antral stage, most follicles undergo atresia (136).
Figure 2-4. Stages of ovarian follicles (136). A) Primordial follicles. B) Primary follicle. C) Secondary follicle. D) Antral follicle. E) Graafian follicle. Figure created by using
https://smart.servier.com/smart_image/ovary/
Primordial follicles are continuously recruited from a resting pool into a pool of growing follicles (Figure 2-5). This recruitment process begins at follicle formation before birth and continues throughout the reproductive life. During the recruitment, intra-ovarian and other unknown factors stimulate a selection of the primordial follicles to grow and develop into antral follicles (136). Most of the antral follicles will eventually undergo atresia. However, at puberty a cyclic recruitment of a few antral follicles starts and takes place at each menstrual
Zona Pellucida Antrum
Flat epithelial cells Granulosa cells Primary oocyte
Theca cells
A B C D E
cycle. Among this group of antral follicles, one leading follicle grows faster than the others and develops into a Graafian follicle. The dominant Graafian follicle ovulates to release the mature oocyte for fertilization. The empty follicle forms the corpus luteum that produces progesterone during early pregnancy. If no fertilization occurs, the oocyte will degenerate within 24 hours (136).
Ovarian follicle loss from birth to menopause
Several statistical models have been developed to describe the decline in number of ovarian follicles during women’s lifespan (120, 122, 123, 137-139). As described above, women are born with 1-2 million follicles in the ovaries, and less than 1000 follicles remain at
menopause (120-123). Yet, the pattern of the ovarian follicle loss is debated (140). The most used model to describe the pattern of follicle loss is the biphasic model, suggesting an
acceleration of follicle atresia around the age of 37 years (120). A more recent model suggests that there is no sudden acceleration of follicle atresia, but rather a gradual increase in follicle atresia with increasing age (139). Nevertheless, atresia of the ovarian follicles starts already in fetal life and continues throughout women’s reproductive life. Although some recent findings suggest that follicular regeneration could occur in adult life, such regeneration is believed to be negligible (141-143). Thus, the timing of menopause is likely a result of a woman’s initial number of ovarian follicles and the rate of the ovarian follicle atresia (136, 138, 140, 144).
Figure 2-5. Recruitment and atresia of ovarian follicles (136). Figure created by using https://smart.servier.com/smart_image/ovary/.
2.3.3The female sex hormone regulation
In women, the hypothalamic-pituitary-gonadal axis refers to the communication between the hormones produced in the hypothalamus, the pituitary gland, and the ovaries (145). The hormones of the axis are the main hormones responsible for regulating reproduction centrally and peripherally. The centrally produced hormones include gonadotropin-releasing hormone (GnRH) from hypothalamus, the luteinizing hormone (LH) and follicular stimulating hormone (FSH) from the pituitary gland. The peripherally produced hormones include estrogens, progesterone, and inhibins that are primarily produced in the ovaries, while activins and follistatin are produced in all tissues, including the ovaries. All of these hormones are regulated by a complex feedback loop. Activins from the peripheral tissues stimulate GnRH secretion from the hypothalamus that stimulates the anterior pituitary to secrete LH and FSH into the bloodstream. In turn, these hormones bind to receptors in the ovaries and stimulate production of estrogens, progesterone and inhibin. Estrogens, progesterone and inhibin have a
A. Continuous recruitment starting before birth B. Cyclic recruitment starting at puberty
Follicle atresia
Antral follicles Graafian follicle
Ovulation Secondary follicles
Primary follicles
Follicle atresia
Resting pool of primordial follicles Continuous depletion throughout the reproductive life
negative feedback on the hypothalamus and the pituitary gland, resulting in a decrease in LH and FSH secretion (146).
Hormonal regulation from menarche to menopause
At puberty, the hypothalamic production of GnRH starts as pulsatile secretion. This hormone further stimulates the pituitary gland to produce LH and FSH. In turn, LH and FSH stimulate an increase in the production of ovarian estrogens that promotes maturation of the ovarian follicles. As described above, one follicle will gain dominance (the Graafian follicle). The Graafian follicle produces higher levels of estrogens and inhibins that suppress further FSH release from the pituitary gland. This FSH suppression causes atresia of the remaining recruited antral follicles. Increasing levels of estrogens stimulate uterine endometrial
proliferation and eventually cause a surge of LH production by the pituitary gland (Figure 2-6 A). This LH surge causes ovulation of the Graafian follicle, and if no fertilization is present, results in the first menstrual bleeding (menarche) (125). Menstrual bleeding is a consequence of a rise in estrogen levels that stimulates proliferation of stroma and glands and causes
elongation of spiral arteries of the uterus (147). The lining of the endometrium thickens. In the absence of pregnancy, and with the accompanying drop in both estrogens and progesterone, shedding of the endometrium occurs resulting in menstrual bleeding. Early menstrual life is characterized by anovulatory cycles, and the menstrual cycles are often irregular through the adolescence (148). The normal length of menstrual cycles ranges from 21-34 days (149), and a woman’s normal cycle length is usually established around six years after menarche (148).
At menopause women lose their menstrual bleedings, which reflects cessation of ovulation due to the loss of ovarian follicles (19). In turn, this results in reduced production of estrogens in the ovaries, which lead to a loss of the hypothalamic feedback inhibition. This causes an increase in GnRH, LH and FSH production as an attempt to increase the estrogen production (Figure 2-6 B). The lack of negative feedback from the ovary is therefore responsible for the unopposed elevation of GnRH, LH and FSH, leading to high concentrations of LH and FSH around menopause (150).
Menopausal symptoms
Menopausal symptoms may occur before the last menstrual period. During the early
menopausal transition, the cycles are usually ovulatory with fluctuating estrogen levels and increasing FSH levels (Figure 2-6 B), and symptoms are generally mild. The late menopausal
transition is characterized by shorter cycle lengths, anovulatory cycles and more prominent menopausal symptoms (3).
The most frequently reported menopausal symptoms are vasomotor symptoms, such as night sweats and hot flushes (151). Hot flushes usually occur in the late perimenopause and the first postmenopausal years. Sleep disturbances are also very common during the menopausal transition and they are mainly connected to frequent awakening due to night sweats (152). Hot flushes and night sweats will usually improve over time. Since estrogen levels decrease, the tissue lining the vagina, vulva, bladder and urethra undergo atrophy, and vaginal dryness, painful intercourse, vulvar itchiness, burning and discomfort may develop (153). Unlike hot flushes and night sweats, these symptoms persist throughout postmenopausal life (3).
Importantly, the decrease in estrogen levels around and after menopause is associated with increased risk of cardiovascular disease and osteoporosis (153).
Since women with early menopause have a shorter duration of cumulative exposure to estrogens during the lifetime, early menopause is associated with increased risk of
cardiovascular disease, osteoporosis and dementia (5-7, 36-38).Conversely, late menopause has been associated with increased risk of hormone related cancers, such as breast (39), endometrial (40) and ovarian cancers (41). Thus, knowledge about factors associated with the timing of menopause and changes in mean age at menopause over time is important for the understanding of the disease burden in the female population and for the individual woman.
Figure 2-6. A) Hormone levels during a normal menstrual cycle. B) Hormone levels in the years before and after menopause. Adapted from Davis SR, Lambrinoudaki I, Lumsden M, Mishra GD, Pal L, Rees M, et al. Menopause. Nature Reviews Disease Primers.
2015;1(1):15004 (3).
3 Background for the present studies
3.1 Temporal trends in age at menarche and menopause
Is mean age at menarche changing?Menarche roughly defines the start of a woman’s reproductive period, and age at menarche displays considerable variation between girls, across countries and time periods (119, 154). In Norway, mean age at menarche decreased from approximately age 16 to 13 years among women born during the years 1830 to 1960 (155). This study was based on records from the earliest established maternity hospitals in Oslo, Bergen and Trondheim. Also in other
countries in the Western world, mean age at menarche decreased from approximately age 17 to 14 years among women born in the early years of 1800s until the 1950s (156). The data on changes in age at menarche is uncertain among women born after the 1950s (119). Some studies suggest that age at menarche has continued to decrease after 1950 (112, 157, 158).
However, others suggest that the downward trend has leveled off (84, 159-161).
Is mean age at natural menopause changing?
The temporal trend in age at menopause are less studied. An increase in mean age at natural menopause has been suggested by a few studies (4, 91, 110, 111). A study of 23 580 women in the North-Trøndelag Health Survey reported a mean age at menopause of 50.6 years (95%
CI: 50.5-50.7 years) and the results suggest that age at menopause increased in Norway across the birth cohorts 1925 to 1977 (91). Another study of 1017 women from Gothenburg in Sweden, suggests that age at menopause increased 0.1 years per birth year among women born during the years 1908-1930 (110). Also a study from the USA supports an increase in age at menopause among women born early in the 20th century, and report that age at
menopause increased from 49.1 years in the 1915 birth cohort to 50.5 years in the 1939 birth cohort (111). However, none of these studies were nationwide or included women born in recent birth cohorts. In addition to the Norwegian study from North-Trøndelag, only two prior studies have studied temporal trends in age at menopause in recent birth cohorts (4, 107). One of these studies included 5288 women from nine European countries born during the years 1940-1973 (4). This study suggested that women in the younger birth cohorts were more likely to reach menopause at an older age compared to women in the older birth cohorts. The effect was, however, heterogeneous across the nine participating countries. The other study,
on the other hand, found that age at menopause among 156 006 postmenopausal women in Portugal, increased across the birth cohorts from 1900 to 1932, but decreased across the birth cohorts from 1933 to 1963 (107). This study excluded women who still had menstrual
periods. Thus, women with early menopause may have been overrepresented, particularly in the most recent birth cohorts. Such overrepresentation of women with early menopause may have resulted in an underestimation of mean age at menopause. Thus, there is a need for nationwide studies of temporal trends in age at menopause in recent generations using appropriate statistical methods.
Is the mean duration of the reproductive period changing?
The time interval between menarche and menopause roughly defines the duration of women’s reproductive period. If mean age at menarche is stable or decreasing and mean age at
menopause is increasing, then the mean duration of the reproductive period is increasing. One study from the USA reported that the number of years between menarche and menopause increased across birth cohorts from 1910 to 1939 (111). However, no studies have
investigated the recent trends of the duration of women’s reproductive period.
A long reproductive period implies high cumulative exposure to estrogens and progesterone that plays an important role in the development of certain endometrial cancers (162-165) and breast cancers (8, 162). In contrast, a long reproductive period has also been associated with a reduced risk of cardiovascular disease (6), osteoporosis (7), dementia (166) and with
longevity (5, 6). Changes in the duration of the reproductive period may therefore contribute to temporal population changes in women’s risk of disease and early death (5, 165, 167).
Thus, valid knowledge about any changes in mean age at menarche and menopause over time is important for the understanding of the changes in disease burden in the female population.
The first objective of this thesis was therefore to investigate possible temporal changes in mean ages at menarche and natural menopause, and in the mean duration of the reproductive period.
3.2 Number of childbirths and age at menopause
The “oocyte-sparing” hypothesis
It is generally accepted that new oocytes are not developed after the fetal period, and that there is a continuous depletion of oocytes from birth until menopause (168). Thus, any factor that influences the rate of follicle depletion could influence age at menopause. Since
pregnancy interrupts ovulation, a high number of childbirths have been suggested to delay menopause (11, 18, 136). During each menstrual cycle, primordial follicles are recruited for development and subsequent degeneration. The number of follicles recruited during the menstrual cycle varies largely between women; from 100 to more than 7000, and the number decreases with age (123). During pregnancy, such monthly recruitment of follicles is
suggested to be suppressed by the high levels of progesterone (169), which possibly could lead to preservation of the primordial follicle reserve (“sparing” of oocytes), and thereby delay menopause (136). Yet, the anticipated suppression of follicles by progesterone during pregnancy has been demonstrated in animals, only (169-171). If the ovarian reserve were less susceptible to follicle atresia during pregnancy, one would expect age at natural menopause to increase by increasing number of childbirths.
Does age at natural menopause increase by number of childbirths?
Several studies suggest that nulliparous women reach menopause earlier than parous women (14, 61, 65, 87). Studies have also reported that women with two childbirths reach menopause later compared to women with no or one childbirth (11, 14, 51, 56, 87, 172). Beyond two childbirths, the association with age at menopause remains inconclusive (11, 15). The association with age at menopause of any number of childbirths beyond four has not been reported, possibly due to lack of statistical power of previous studies (14, 15, 46, 56, 87, 173).
A study from China of 33 054 women shows in their age-adjusted model that women with three or more childbirths actually may reach menopause earlier than women with two childbirths. Also, some studies applied linear regression models for their data analyses, and number of childbirths was included as a continuous variable (46, 83). In such models, a non- linear association of number of childbirths with age at menopause cannot be detected. Thus, it remains uncertain whether age at menopause increases linearly with number of childbirths.
Identification of factors that are associated with the timing of menopause may improve our understanding of the biological mechanisms that ultimately lead to menopause. The previous
studies reporting that age at menopause increases with increasing number of childbirths have had methodical limitations in design, analytic approach and/or lacked statistical power to study women with a high number of childbirths. Therefore, the second objective of this thesis was to study the association of number of childbirths with age at natural menopause.
3.3 Childbirth close to natural menopause
It has been assumed that a successful spontaneous pregnancy is rarely achieved within the 10- year interval prior to menopause (1, 174, 175), and that such interval is independent of age at menopause (120, 176). Women’s mean age at first childbirth has increased in worldwide (19- 21), and age at menopause varies widely (4, 42). Postponing pregnancy until the age of 35-40 years may therefore reduce the woman’s chance of giving birth, particularly if she will experience early menopause.
The general assumption that women cannot achieve a successful spontaneous pregnancy within 10 years before natural menopause is mainly based on retrospective data by comparing the age distribution at last childbirth in one population with the age distribution at menopause in another (1, 177-179). One of these studies compared the distribution of age at last
childbirth in a 19th century Canadian natural fertility population with the distribution of age at menopause in a Dutch population of women born during the years 1911-1925 (1). The shape of the distributions of age at last childbirth and age at menopause were almost identical, and the mean difference between the age distributions was 10 years. Importantly, they did not have access to individual data and no previous population studies of childbirth in the years close to menopause have used individual data.
Since modern women delay childbirth (20, 21), knowledge about their chances of having a successful spontaneous pregnancy in the years close to menopause is needed. Thus, the third objective of this thesis was to study women who gave birth in the years close to natural menopause, and to assess whether a successful pregnancy close to menopause depends on age at menopause.
4 Objectives for the thesis
Paper I. To investigate temporal changes in mean age at natural menopause. We also investigated temporal changes in mean age at menarche and in the time interval between menarche and natural menopause.
Paper II. To study the association of number of childbirths with age at natural menopause.
We also explored the shape of the association and allowed for a non-linear relation in the data analyses.
Paper III. To study the proportions of women who had a successful pregnancy within 5 years and within 10 years prior to natural menopause. We also studied whether the proportions of women with a successful pregnancy within 5 years and within 10 years prior to natural menopause depended on age at menopause.
5 Material and methods
5.1 The BreastScreen Norway
Paper I and Paper II are retrospective population studies of women who participated in the Norwegian breast cancer screening program (BreastScreen Norway) (180). The BreastScreen Norway was established in 1996 and is administrated by the Cancer Registry of Norway. At that time breast screening was offered in 4 out of 19 Norwegian counties (181). The program expanded and became nationwide in 2005. Since then, the program has offered breast cancer screening every second year to all women at the age of 50-69 years in Norway. The overall participation rate in the screening program is 75%, and women older than 60 years are more likely to participate than younger women (182).
During the years 2006-2015, all women who attended the BreastScreen Norway were invited to answer self-administered questionnaires (180). The aim was to collect information about risk factors for breast cancer and identify women at high risk (183). Two questionnaires were sent by post along with the invitation to the breast cancer screening examination, and they were returned at the screening site. The first questionnaire included questions about
demographics, reproductive factors and lifestyles prior to the age of 50 (See Appendix). This questionnaire was administered the first time the women attended the screening, only. The second questionnaire included questions about current health, menstruation, and surgery on the uterus or ovaries (See Appendix) and was distributed at all invitations to the screening in the period (every second year) (184). In total 611 711 women attended the screening program during the years 2006-2015. Of these, 64% answered the first questionnaire and 87%
answered the second questionnaire (183). We used the data that was collected during the years 2006-2014, and 538 892 women answered one of the questionnaires (Figure 5-1). Altogether 392 238 women answered both questionnaires.
Figure 5-1. Overview of participants who had answered the questionnaires during the years 2006-2014.
5.1.1 Paper I – Study sample, variables and statistical analyses
Aim of the study: To study if mean age at menarche, mean age at natural menopause and mean number of years between menarche and natural menopause changed from the 1936 birth cohort to the 1964 birth cohort.
5.1.1.1 Study sample
All women, born during the years 1936-1964, and who had completed both questionnaires were eligible to our study population (N=387 273) (Figure 5-2). We excluded 155 women who reported that menstruation never occurred, women with missing information on age at menarche (N=23 590) and women with reported age at menarche less than 5 years or above 25 years (N=1414), since we considered these values to be outlying. We also excluded women with missing information on age at menopause (N=27 276) and women with age at menopause less than 15 years or above 71 years (N=1078). Additionally, we excluded 1101 women with missing information or outlying values on age at hysterectomy and/or bilateral oophorectomy. Since mean age at menarche and menopause shows geographical variation (4,
185), we excluded women who were not born in Norway or lacked information about country of birth (N=19 682). Hence, a total of 312 656 women could be included in the study.
Figure 5-2. Flow chart of the study sample in paper I.
Eligible participants
Women born 1936-1964 who responded to the questionnaires N=387 273
155 participants who reported that menstruation never occurred.
N=387 118
25 275 participants (6.5%) without information about age at menarche (N=23 590) or with outlying values (N=1414).
N=361 843
28 404 participants (7.8%) without information about last menstruation (N=13 195), age at last menstruation (N=14 131) or with outlying values (N=1078).
N=333 439
1101 participants (0.3%) without information about age at hysterectomy (N=585) and/or bilateral oophorectomy (N=451).
N=332 338
Final study sample N=312 656
19 682 participants (5.9%) without information about country of birth (N=2073) or born outside Norway (N=17 609).
5.1.1.2 Variables Age at menopause
The information about age at menopause was based on the following two questions in Q2:
‘Are you still having menstrual periods?’ (yes/ yes, but irregularly/ no) and ‘If you no longer have menstrual periods, how old were you at your last menstrual period?’ In the main
analyses, age at menopause was used as a continuous variable. For descriptive purposes, we also categorized age at menopause into menopause before the age of 45 (early menopause, yes/no), and menopause before the age of 40 (primary ovarian insufficiency, yes/no).
Age at menarche
Age at menarche was based on the following question in Q1: ‘At what age (years old) did you have your first menstrual period?’ Age at menopause was used as a continuous variable.
Women’s year of birth
In the main analyses the women’s year of birth was used as a continuous variable, but was also categorized as follows: 1936-1939 (reference), 1940-1944, 1945-1949, 1950-1954, 1955- 1959, and 1960-1964.
5.1.1.3 Statistical analyses
By only including postmenopausal women, an overrepresentation of women with early menopause may occur, particularly in the most recent birth cohorts. Such overrepresentation of women with early menopause may result in an underestimation of mean age at menopause.
To avoid underestimation of age at natural menopause in the most recent birth cohorts, we therefore used survival analyses. In analyses of age at menopause according to birth year, the follow-up time was from birth until attained age at menopause. The women who were still having menstrual periods or had irregular menstrual periods contributed with follow-up time until their attained age at data collection (censoring). The women who reported hysterectomy, bilateral oophorectomy or both surgeries prior to menopause, contributed with follow-up time until their attained age at surgery. The same approach was used when estimating the trend in the number of years between menarche and natural menopause, but in these analyses follow- up time was from menarche until menopause or censoring. When estimating age at menarche according to birth year, follow-up time was from birth until menarche, and all women
contributed with follow-up time until menarche.
We estimated the mean age at menarche, natural menopause and number of years between menarche and natural menopause according to birth year (as a continuous variable and in five- year intervals) by applying flexible parametric survival models. Also the association of birth year with age at menarche, age at natural menopause and number of years between menarche and menopause was estimated as crude hazard ratios by applying flexible parametric survival models. We calculated 95% confidence intervals (CI) for the estimated restricted means and for the hazard ratios. By using restricted cubic splines with 4 degrees of freedom (5 knots), we allowed for possible non-linear trends.
In additional analyses, we studied the associations within levels of completed education (less than high school, high school, college/university), and in a subsample where women with menopause before the age of 40 years and after the age of 60 years were excluded.
5.1.2 Paper II – Study sample, variables and statistical analyses
Aim of the study: To study the association of number of childbirths with age at natural menopause.
5.1.2.1 Study sample
A total of 392 238 women, aged 50-69 years, who had completed both questionnaires were eligible to our study population (Figure 5-3). We excluded 157 women who reported that menstruation had never occurred. Women with missing information on number of childbirths (n=54 615) or with more than 19 childbirths (N=251) were excluded. Also in this study we excluded women with missing or outlying information on age at menopause (n=30 184), and with missing information about hysterectomy and/or bilateral oophorectomy (n=888). Thus, a total of 310 147 women could be included in the study.
Figure 5-3. Flow chart of the study sample in paper II.
5.1.2.2 Variables Age at menopause
Age at menopause was defined and analyzed as described in Paper I.
Number of childbirths
The number of childbirths was based on the following questions: “Have you ever had a pregnancy that lasted longer than 6 months?” (yes/no), and “If yes, how many?” Number of
Eligible participants
Participants responding to the questionnaires N = 392 238
157 participants who reported that menstruation never occurred.
N=392 081
54 615 participants (13.9%) without information about number of childbirths (N=54 364) or with outlying values (N=251).
N=337 466
30 184 participants (8.9%) who did not answer the questions about last menstruation (N=11 805), age at last menstruation (N=13 634) or had outlying values (N=992).
N=311 035
888 participants (0.3%) who did not report age at hysterectomy (N=504) and/or bilateral oophorectomy (N=415).
Final study sample N=310 147
childbirths was categorized as follows: 0, 1, 2, 3 (reference), 4, 5, 6, and ≥7 childbirths. We also used number of childbirths as a continuous variable (0-19 childbirths).
Other study variables
All analyses were adjusted for the women’s year of birth. We also made adjustments for the below factors since they may be associated with number of childbirths (21, 186-189) and age at menopause (4, 15, 42, 55). The following variables were categorized as follows: Cigarette smoking (current smoker, former smoker, or never-smoker), educational level (< high school, high school, ≤ 4 years of college/university, or > 4 years of college/university), country of birth (Norway, other countries in Europe, or countries outside Europe). Oral contraceptive use was dichotomized into “ever or never”. The woman’s year of birth and body mass index were used as continuous variables.
5.1.2.3 Statistical analyses
Also in paper II we applied survival analyses to avoid underestimation of age at natural menopause, since not all women had reached menopause at the time of data collection. We used survival analyses to estimate mean age at menopause with 95% confidence intervals (CI), and median age at menopause with interquartile ranges (IQR) according to number of childbirths. The follow-up time was from the woman’s birth until age at menopause or
attained age at data collection (censoring). Thus, the women who still had menstrual cycles or had irregular menstrual cycles, contributed with follow-up time until their attained age at data collection. Also the women who had undergone hysterectomy, bilateral oophorectomy, or both surgeries prior to menopause were censored, and these women contributed with follow- up time until their attained age at surgery.
The association of number of childbirths (as a categorical variable) with age at menopause was estimated as hazard ratios (HR) by applying Cox proportional hazard models. We performed additional analyses, using number of childbirths as a continuous variable (0-19 childbirths), and in this analysis we applied a Cox proportional hazard model with restricted cubic splines to allow for a non-linear association. The knots were placed at the 10th, the 50th, and the 90th percentile of the childbirth distribution (1, 2, and 4 childbirths). All analyses were adjusted for the women’s year of birth. In additional analyses, we also made adjustment for cigarette smoking, educational level country of birth, oral contraceptive use and body mass index. We repeated the above analyses among women who had never used hormonal
intrauterine device and/or systemic menopausal hormone therapy, and among women born before 1950 and among women born in 1950 or after.
Table 5–1. Summary of variables used in Paper I and II.
Exposure variables Outcome variables Covariates Paper I Woman’s year of birth Age at menarche
Age at menopause
Number of years between menarche and menopause
Paper II Number of childbirths Age at menopause Woman’s year of birth Smoking habits
Educational level Country of birth Oral contraceptive use Body mass index
5.2 The HUNT2 Survey
In Paper III, we performed a retrospective study with data obtained from the second Nord- Trøndelag Health Survey (HUNT2 study). The HUNT2 study is part of a population database with both questionnaire data and clinical data from inhabitants aged 20 years or older in the Nord-Trøndelag county in Norway. So far, four health studies have been completed; HUNT1 (1984-1986), HUNT2 (1995-1997), HUNT3 (2006-2008) and HUNT4 (2017-2019) (190).
The purpose of the study is to provide data to researchers who study medical and health related topics. The large number of participants and the magnitude of data collected from each participant make the HUNT study one of the largest health studies in Norway (191).
The second Nord-Trøndelag Health Study (HUNT2 Survey) took place during 1995-1997, and constituted both a new cross-sectional survey and a follow-up of HUNT1 Survey (192).
The overall participation in the HUNT2 Survey was 71.2%, and participants in the age group 60-69 years was more likely to participate than the younger and older participants. More women than men participated (75.5% versus 66.7%), and in our study we included the
women. We used data collected by two self-administrated questionnaires (See Appendix). The first questionnaire was sent by mail together with invitation letter to all women aged 20 years or older (192). This questionnaire included questions about socio-demographic factors and health (See Appendix) and was completed before and returned at the day of attendance. Of all women, 71% answered the first questionnaire. The second questionnaire included questions about menstruation, childbirth and surgery on the ovaries and the uterus (See Appendix). It was handed out at the day of attendance to women aged 20-69 years, only, and returned by mail. Of women aged 20-69 who had answered the first questionnaire, 87% also completed the second. Thus, 60% of women in this age group in the Nord-Trøndelag county answered both questionnaires. During 1995-1997 the county was mostly rural and sparsely populated (192). In general, the women had many children and limited access to modern contraceptive methods. Thus, data from HUNT2 was appropriate to use in our study since our aim was to include women of a natural fertility population.
5.2.1 Paper III – Study sample, variables and statistical analyses
Aim of the study: To study the proportions of women who had a successful spontaneous pregnancy within 5 years and within 10 years prior to natural menopause, and if the proportions differed by age at menopause.
5.2.1.1 Study sample
All women aged 20-69 years, who had answered both questionnaires (N=24 865 women) were eligible to our study population (Figure 5-4). We excluded women who had not given birth (N=3554), women with missing information about the number of childbirths (N=740) and women who had not reached menopause (N=12 722). We also excluded women with missing or implausible values for age at last childbirth or age at menopause (N=459).
Additionally, we excluded women who had hysterectomy and/or bilateral oophorectomy before natural menopause, or had missing information about age at such surgery (N=1110).
To avoid an overrepresentation of women with early menopause, and to include women with
limited access to modern contraceptive methods, we excluded menopausal women born after 1940 (N=2122). Thus, 4157 women born during the years 1925-1940 could to be included in the study.
Figure 5-4. Flow chart of the study sample in paper III.
Exclusion of 12 722 women who had menstrual periods (N=12 630) or an ongoing pregnancy (N=570).
Exclusion of 459 women without information about age at last menstrual period (N=392) or with outlying values on time interval between last childbirth and menopause (N=67).
Women in the HUNT2 Survey
All women aged 20-69 years responding to both questionnaires.
N=24 865
Exclusion of 1110 women without information about age at bilateral oophorectomy (N=42) or hysterectomy (N=36), or who had undergone bilateral oophorectomy (N=316) and/or hysterectomy (N=1017) before
menopause.
N=20 571
N=7849
N=7389
Exclusion of 4294 women who are nulliparous (N=3554) or missing values on number of.childbirths (N=740).
Exclusion of 2122 women born after 1940.
N=4157 N=6279
5.2.1.2 Variables
Number of previous childbirths and age at last childbirth
Information about the number of previous childbirths and age at last childbirth was based on the following questions: “How many times have you given birth?”, “List the year of each childbirth” and “How old were you at your last childbirth?” In additional analyses, number of previous childbirths was categorized as 0-1, 2-3 and > 3 childbirths.
The time interval between last childbirth and menopause
The time interval between last childbirth and menopause was calculated by subtracting age at last childbirth from age at menopause and categorized as childbirth within 5 years (yes/no) and childbirth within 10 years (yes/no) prior to menopause.
Age at menopause
Information about age at menopause was obtained using the following questions: “Do you still have menstrual periods?” (yes/no) and “If no, at what age did you have your last
menstrual period?”. Age at menopause was categorized as <45, 45-49, 50-54 and ≥ 55 years.
Table 5–2. Study variables used in Paper III Study variables
Paper III Childbirth within 5 years prior to menopause Childbirth within 10 years prior to menopause Age at menopause
Number of previous childbirths
5.2.1.3 Statistical analyses
We used kernel density estimation to illustrate the age distributions at last childbirth and age at menopause. Within the above-defined categories of age at menopause, we calculated the mean number of childbirths, mean age at last childbirth and mean age at menopause.
Differences in the means between the categories were assessed by analysis of variance (ANOVA). A 5% level of statistical significance was chosen for all analyses.
Thereafter we calculated the proportions of women who gave birth within 5 years and within 10 years before menopause among all women, and within the categories of age at menopause.
We repeated these data analyses after excluding women who had used oral contraceptives or had undergone sterilization. We also performed separate analyses among women born in 1930 or earlier.
In supplementary analyses, we calculated the proportions of women with childbirths within 5 years and within 10 years prior to menopause according to the number of previous childbirths, and we repeated these analyses within the categories of age at menopause.
