A full term placenta has a diameter of 15-25 cm, is about 3 cm thick and weighs 500-600 g (26). For the placenta to start developing, the endometrium must first be receptive for implantation of a blastocyst. Six to seven days after fertilization the embryo implants in the uterine wall, and will be in contact with the endometrium, myometrium and uterine vasculature. The cells necessary for developing a placenta are created shortly after
fertilization, and will continue developing until a full grown placenta is in place (25).
Placentas are developed partly from maternal and partly from fetal tissues, which are respectively called decidua and chorion. Figure 1 shows a section of a pregnant uterus illustrating various anatomical features. Each release of an ovum is accompanied by the formation of decidua. When the ovum is fertilized, the decidua is further developed and becomes the maternal side of a placenta. The decidua is formed from the uterine mucous membrane, which is part of the endometrium. The mucous membrane undergoes
change, and becomes more vascularized and enlarged. The location where the ovum is attached to the decidua is termed decidua serotina, and from it the maternal part of the placenta is developed (27). However the chorion, the fetal side of the placenta, is developed from trophoblasts (28). The trophoblasts ensure development of the villi that form the fetal-maternal interface (25). From the outer surface, several villi of chorion develop and invade the decidua of the uterus. Here nutrients are absorbed to further
2 Theory
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develop the embryo. The villi that invade the decidua serotina increase greatly in size and complexity and constitute the chorion frondosum, which later becomes the fetal part of the placenta (27). It can be said that the decidua and myometrium are the maternal side, and the remaining part the fetal side, which is an important distinction for later in the method chapter. The placenta can implant itself in different locations along the anterior, posterior, fundal (top) or lateral uterus walls (5, 29).
There is a complex circulatory system in place to ensure the wellbeing and development of the fetus, where the placenta receives about 17% of the maternal cardiac output (6).
The placental development starts approximately 6 to 7 days after conception (30).
Around the 17th day after conception, both fetal and maternal blood vessels are
functioning and placental circulation is established (25). Its circulation differs from other organs in our body, as it is the only organ with two separate blood supplies that come from two separate organisms: the mother and the fetus. It is however important to note that the separate blood supplies never mix, all exchange between the mother and fetus happens via diffusion (6).
Figure 1: Illustration showing a section of a pregnant uterus in the third and fourth month depicting decidua serotina and villi of chorion frondosum, which are important in placental development. The image shows an early stage in the development of a full grown placenta and fetus (27).
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The placenta has a rough side and a smooth side, see Figure 2. On the rough maternal side, of the placenta there are several divisions, these are the cotyledons. Inside a cotyledon, the intervillous space and villi can be found. The villi branch out like trees to maximize transfer of nutrients and waste products. The roughness of the maternal side helps anchor the placenta, along with anchoring villi. However, the fetal side of the placenta is smooth and fetal vessels can be seen radiating from the umbilical cord that is attached in the central part of the placenta, see Figure 2 (6).
Figure 3 illustrates how the placental circulation is organized. The umbilical cord is the fetus’s connection to the placenta, which is in turn anchored to the mother. Each cotyledon is supplied by a branching artery and vein from the main umbilical vessels, which in turn branch out in villi. It is within the placenta’s intervillous space that the diffusional transfer takes place. The maternal blood enters the placenta via arterioles that empty into this space, and is drained by venous sinuses (6). The fetus’s connection to the placenta is via two arteries and one vein within the umbilical cord. The umbilical cord’s blood vessels branch out into villi, finally forming a capillary network, that are bathed in the mother’s blood in the intervillous space of the placenta (25). The
organization of the placenta’s circulation maximizes the transfer of nutrients and waste products (6). To ensure satisfactory placental and fetal growth, metabolism and waste removal, an adequate perfusion of the placental intervillous space is necessary. It is estimated that the uteroplacental blood flow increases during a pregnancy from 500 to 700 ml/min. In perspective, the blood flow of the entire circulation of a non-pregnant woman is approximately 5000 ml/min (25).
The placenta’s unique anatomy and physiology allows it to sustain the fetus throughout a pregnancy. During the pregnancy the placenta will grow continuously in both
circumference and thickness until the end of the 4th month. The increase in thickness is due to the increasing size and length of the villi and expansion of the intervillous space.
The placenta will not increase in thickness after the 4th month, but it will continue to grow in circumference throughout the pregnancy (6). The placenta is in use throughout the entire pregnancy, and will grow continuously with the fetus (28). The placenta grows more rapidly than the fetus in the first trimester, and by 17 postmenstrual weeks the fetus and placenta weight approximately the same. By term the placenta weighs about 1/6 of the fetus (25).
Figure 2: Images of a placenta after birth, showing its maternal (left) and fetal (right) side (6).
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Placental volumes vary throughout a pregnancy, and also vary depending on fetal weight. Using 3D US the mean placental volume has been shown to range from 86 cm3 at 12 weeks to 427 cm3 at 40 weeks (29). It has however been shown that US
significantly underestimates placental volume after the first trimester. A study done with MRI, showed that mean placental volume from week 6 to 39 was 10.1 cm3 and
1039 cm3, respectively (31). In Table 1 and 2 placental volumes for week 25-27 are shown for US and MRI, since these weeks are relevant for this study. The tables are from two different studies, and therefore the data is presented differently. Placental volume estimates can be important because smaller or larger placentas can indicate possible disorders of the placenta (3, 4).
Figure 3: Illustration showing how both maternal and fetal circulation are connected to the placenta. The fetus’s connection is via the umbilical vein and arteries, while the mother’s is via the endometrial arteries and veins. The maternal side (decidua and myometrium) and fetal side (chorion) of the placenta are shown, in addition to the intervillous spaces.
Table 1: Shows percentiles of placental volume depending on the gestational age. The study used 3D US (29).
Table 2: Shows placental volume depending on gestational age. The study used MRI.
SD=standard deviation (31).
Gestational age
Percentile of placental volume (cm3)
10th 50th 90th 25 135.9 243.1 361 26 141.9 255.4 379.8 27 147.9 267.7 398.7
Gestational age
Placental volume (cm3) Range Mean SD 25-27 259-637 460.8 89.3
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