The amniotic sac is the bag in which the embryo and fetus develops. Its inner layer is the amnion. Its outer layer is the chorion. The chorion becomes the fetal part of the placenta. The amnion secretes amniotic fluid into an amniotic cavity in which the embryo floats by day 12. At this time, the temporary yolk sac begins to form.
Chorionic villi are finger-like projections of the chorion into the wall of the uterus. They provide a large exchange surface between the embryonic and maternal circulations by day 13. At this time, the developing embryo is connected to extra-embyonic tissues by a connecting stalk (allantois). By the third month, this connection becomes the umbilical cord. Its two arteries and one vein connect the fetus to the placenta. The placenta has both fetal and maternal components. It allows nutrient, waste, and gas exchange, without direct mixing of the blood of the mother and the fetus.
The corpus luteum maintains hormone production if fertilization occurs, until placenta takes over after month 4. hCG (human chorionic gonadotropin) is secreted by the trophoblast (days 5-7), chorion (day 8 to month 4) and the placenta after that. This is the hormone detected by pregnancy tests.
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