The placenta is a fetomaternal organ. The fetal portion of the placenta is known as the villous chorion. The maternal portion is known as the decidua basalis. The two portions are held together by anchoring villi that are anchored to the decidua basalis by the cytotrophoblastic shell.
The endometrium (lining of the uterus) of the mother is known as the decidua (“cast off”), consisting of three regions named by location.
Table 7 - Regions of the Decidua
As the embryo enlarges, the decidua capsularis becomes stretched and smooth. Eventually the decidua capsularis merges with the decidua parietalis, obliterating the uterine cavity.
The placental membrane separates maternal blood from fetal blood. The fetal part of the placenta is known as the chorion. The maternal component of the placenta is known as the decidua basalis.
· Oxygen and nutrients in the maternal blood in the intervillous spaces diffuse through the walls of the villi and enter the fetal capillaries.
· Carbon dioxide and waste products diffuse from blood in the fetal capillaries through the walls of the villi to the maternal blood in the intervillous spaces.
Although the placental membrane is often referred to as the placental barrier, many substances, both helpful and harmful, can cross it to affect the developing embryo.
· Primary chorionic villi are solid outgrowths of cytotrophoblast that protrude into the syncytiotrophoblast.
· Secondary chorionic villi have a core of loose connective tissue, which grows into the primary villi about the third week of development.
· Tertiary chorionic villi contain embryonic blood vessels that develop from mesenchymal cells in the loose connective tissue core. These blood vessels connect up with vessels that develop in the chorion and connecting stalk and begin to circulate embryonic blood about the third week of development.
Figure 4 - Structure of placenta and chorionic villi
Amniotic fluid has three main functions: it protects the fetus physically, it provides room for fetal movements, and helps to regulate fetal body temperature. Amniotic fluid is produced by dialysis of maternal and fetal blood through blood vessels in the placenta. Later, production of fetal urine contributes to the volume of amniotic fluid and fetal swallowing reduces it. The water content of amniotic fluid turns over every three hours.
The umbilical cord is a composite structure formed by contributions from:
· Fetal connecting (body) stalk
· Yolk sac
The umbilical cord contains the right and left umbilical arteries, the left umbilical vein, and mucous connective tissue. Presence of only one umbilical artery may suggest the presence of cardiovascular anomalies.
Fetal circulation involves three circulatory shunts: the ductus venosus, which allows blood from the placenta to bypass the liver, and the ductus arteriosus and foramen ovale, which together allow blood to bypass the developing lungs. Refer to the section on changes at birth for more information on the fates of these structures.
Dizygotic twins are derived from two zygotes that were fertilized independently (i.e., two oocytes and two spermatozoa). Consequently, they are associated with two amnions, two chorions, and two placentas, which may (65%) or may not (35%) be fused. Dizygotic twins are only as closely genetically related as any two siblings.
Monozygotic twins (30%) are derived from one zygote that splits into two parts. This type of twins commonly has two amnions, one chorion, and one placenta. If the embryo splits early in the second week after the amniotic cavity has formed, the twins will have one amnion, one chorion, and one placenta. Monozygotic twins are genetically identical, but may have physical differences due to differing developmental environments (e.g., unequal division of placental circulation).
The fetus implants in such a way that the placenta or fetal blood vessels grow to block the internal os of the uterus. See implantation.
Some erythrocytes produced in the fetus routinely escape into the mother’s systemic circulation. When fetal erythrocytes are Rh-positive but the mother is Rh-negative, the mother’s body can form antibodies to the Rh antigen, which cross the placental barrier and destroy the fetus. The immunological memory of the mother’s immune system means this problem is much greater with second and subsequent pregnancies.
Deficiency of amniotic fluid (less than 400 ml in late pregnancy). It can result from renal agenesis because the fetus is unable to contribute urine to the amniotic fluid volume.