During development, the fetus is nourished by the placenta, an organ formed from the outermost layer of the embryo, the chorion, and the innermost layer of the uterus, the endometrium. Here, exchanges take place between the bloodstreams of the mother and the fetus through fetal capillaries.
The umbilical cord contains the blood vessels that link the fetus to the placenta. Fetal blood is carried to the placenta in two umbilical arteries. While traveling through the placenta, the blood picks up nutrients and oxygen and gives up carbon dioxide and metabolic waste. Restored blood is carried from the placenta to the fetus in a single umbilical vein.
Although the bloodstreams of the mother and the fetus do not mix, and all exchanges take place through capillaries, some materials do manage to get through the placenta in both directions. For example, some viruses, drugs, and other harmful substances are known to pass from the mother to the fetus; fetal proteins can enter the mother's blood and cause immunologic reactions.
During gestation (the period of development), the fetus is cushioned and protected by fluid contained in the amniotic sac (amnion), commonly called the bag of waters. This sac ruptures at birth.
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