1.3.1 Older theories
Throughout the years that NVP has been investigated a plethora of aetiologies for its presence have been hypothesised. These hypotheses, much like hairstyles and clothing, have undergone various trends, so much so that it was at one point termed the ‘disease of theories’ [21]. As the bulk of research performed on the topic has concentrated mainly upon emesis associated with episodes of nausea and very few observing nausea alone, the term ‘NVP’ shall be used in the introduction and discussion section to incorporate both NP and NVP, unless otherwise specified.
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During the late 19th century, in a medical environment excited by Sigmund Freud’s recent psychological publications, NVP was thought to be caused by some imbalance in the mind of the mother to be [21]. Hypotheses regarded NVP as a mental illness, a result of repressed sexuality, or a subconscious act against the foetus, the latter considered especially robust since women tended to stop vomiting in the later stages of gestation after their subconscious had accepted the notion that the infant’s delivery was imminent [21].
Surprisingly, many of these ideas continued and developed into the early part of the 20th century, whereby the mind, or psyche, was still considered to be the source of NVP symptoms. The jargon of this time described NVP’s aetiology as stemming from a neurosis closely allied with hysteria as the primary cause, with hysteria being considered a
manifestation of the woman’s unconscious loathing of her husband and expected child [22].
This loathing was central in the diagnosis made for perhaps NVP’s most famous victim, the 19th century English author Charlotte Brontë, who died from starvation and dehydration after suffering from very severe NVP early in her first pregnancy [23]. In the psychoanalysis written about her, it was stated that Brontë was ‘fearful, conflicted, and reluctant to accept her future marriage and childbearing’, concluding that ‘pernicious vomiting . . . always has psychogenic features’ [24]. Acceptance of the mind’s power to contribute to (if not cause) NVP was still prevalent on the late 20th century. A study performed in the 1970’s found that 50% of obstetricians questioned believed that NVP was a psychologically based malady [25].
It was not until the late 1970s/early 1980s that a new interest in NVP began to emerge, and with this new interest came new hypotheses regarding aetiology. The notion that the NVP symptoms were caused by a slow adaptation from the gestating woman to hormonal
fluctuations associated with pregnancy, or else altered hormonal levels, are among the most popular of the latter theories [20, 26, 27]. Another novel hypothesis introduced in the 1980’s involved abnormal gastro-electric activity in the gestating woman’s gastrointestinal tract - either slower (bradygastrias) or faster (tachygastrias) neural activity, supposedly resulting in symptoms of NVP [28]. These hypotheses dominate much of the present consensus regarding NVP, in addition to the hormonal-related theories.
1.3.2 Hormone-related theories
During early pregnancy, estradiol increases and slowly continues to rise throughout the remainder of pregnancy [29]. Also increasing early in the first trimester is progesterone [30, 31]. Their association with NP and NVP have been proposed by studies reporting that women with an inability to tolerate oral contraceptives have a remarkably high risk for NVP [32-36] .
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Progesterone and estradiols/estrogen have been demonstrated to inhibit the activity of smooth muscles [37], which can lead to delayed gastric emptying, and consequently result in gastric reflux and increased occurrences of gastric emptying [3, 37]. However, these results have not been conclusive, as other studies report no evidence of either progesterone or estrogen’s role in NVP [38, 39].
Human chorionic gonadotropin (hCG) is now probably recognised as the leading cause of NVP. It is mainly accepted as the prime aetiological hormone because of the near-identical timing hCG has with the onset of NVP symptoms [9]. Although secretion of hCG by the placental trophoblasts has been proposed as the most likely endocrine contributor to NVP, data to support this are far from conclusive [29, 40, 41].
One review examining hCG and NVP found an association in 13 of 17 studies
reviewed, with the authors speculating that the failure of other studies to show an association was possibly owing to varying biologic activity of different forms of hCG [38].
1.3.3 Helicobacter pylori
There has been interest generated into the association between the bacteria Helicobacter pylori (H. pylori) and NVP ever since a systematic review addressing HG and H. pylori infection was performed in 2007 and an association was suggested to exist [42]. However, the review mentioned that limitations in individual studies may have a bearing on the differing results [42]. Others have also discussed contraindications possibly resulting from different testing procedures used to determine the presence of H. pylori [43]. Confusing matters further is the knowledge that H. pylori infection symptoms include nausea, vomiting, and heartburn, symptoms also reported to occur in 50 to 90% of all pregnancies [44].
1.3.4 Genetics
Previous research has reported that NVP tends to have a higher frequency in monozygotic twins, in women whose siblings and mothers were affected by NVP, and is correlated with other genetically determined conditions, such as taste sensation, anosmia (loss of smell), and glycoprotein receptor defects [15, 20, 38, 45, 46]. These results have strengthened the hypothesis that NVP is a heritable condition with a genetic aetiology [47].
Further examples supporting this hypothesis are studies showing significantly lower incidences of NVP in samples of American Southern black teenagers when compared with American Southern white teenagers [48], and the incidence of NVP to be slightly lower in samples of South African blacks as compared to South African whites [49].
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