Placenta and Amniotic Fluid Embryology Questions

The placenta is a unique organ, in that it is a composite of tissue from two different sources—
the mother and fetus. The maternal component is the decidua basalis, and the fetal component
is the villous chorion.

The amnion does not pass through the primitive umbilical ring. As craniocaudal folding
occurs, the amnion becomes the outer covering of the umbilical cord.

The normal amount of amniotic fluid at term is 1000 mL. However, the amount of amniotic
fluid at various stages of pregnancy can be indicative of congenital malformations. Oligohydramnios (400 mL in late pregnancy) may be indicative of renal agenesis. Polyhydramnios
(2000 mL in late pregnancy) may be indicative of either anencephaly or esophageal atresia

A placenta implanted in the lower part of the uterus near the internal os is called placenta
previa. The repeated episodes of bright-red vaginal bleeding are caused by the gradual dilation of the uterus in the later stages of pregnancy. As the uterus dilates, spiral arteries and veins
supplying the placenta are ruptured. The mother may bleed to death, and the fetus is placed in
jeopardy because of the compromised maternal blood flow.

Polyhydramnios is associated with the inability of the fetus to swallow because of esophageal
atresia or anencephaly. Polyhydramnios can also result from absorption defects such as duodenal atresia. The inability of the embryo to swallow the amniotic fluid means that the fluid cannot be absorbed into the fetal blood and removed by the placenta and passed into the maternal
blood

A velamentous placenta occurs when umbilical blood vessels abnormally travel through the
amniochorionic membrane before reaching the placenta proper. If the vessels cross the internal
os, a serious condition called vasa previa exists. As the fetus grows during pregnancy and the
amniochorionic membrane stretches, the umbilical vessels may rupture. When that happens,
the fetus will bleed to death. The mother is in no danger of bleeding to death in vasa previa
because only the umbilical vessels rupture.

Insulin, like all protein hormones, does not cross the placental membrane in significant
amounts.

Dizygotic twins and 35% of monozygotic twins have two placentas, two amniotic sacs, and
two chorions (“222”).

Reduced AFP levels are associated with Down syndrome. All of the other defects (neural
tube defects such as spina bifida and anencephaly, omphalocele, and esophageal atresias) are
associated with elevated AFP levels.

The intervillous space contains only maternal blood as the spiral arteries of the endometrium penetrate the outer cytotrophoblast shell.

During the later stages of pregnancy, the placental membrane becomes very thin and consists of two layers—the syncytiotrophoblast and the fetal endothelium.