Amniotic Sac

Within the womb, the fetus moves and floats within the enclosed fluid-filled amniotic sac. Some of the amniotic fluid, which protects and cushions the fetus against physical injury, seeps through the placenta and through the wall of the sac (which is only two cell layers thick) from the uterus. But most of it comes from fetal blood by way of the baby's lungs and kidneys. This sac is more like a circulating bath than a pool. By the time the baby is developed, the sac holds approximately a quart of fluid, of which about a third is recycled hourly. In the latter part of pregnancy, the fetus swallows a good deal (approximately 16.5 ounces) of this clear, sterile liquid daily.