A bubblelike sac called the “amnion” forms around the embryo and the body stalk. This sac is filled with fluid, which serves as a shock absorber and also provides some nourishment for the forming baby.

The Amniotic Sac​—Watery Home During Gestation

During the gestation period the baby lives in the amniotic sac, which is a tough, shiny, transparent membrane. It keeps full of fluid, but this is by no means stagnant. More than a third of its volume is removed and replaced every hour. This amounts to a total daily exchange equivalent to six gallons! In about the third month the baby begins to practice inhaling and exhaling exercises. Breathing in the amniotic fluid may help in formation of its lung sacs. The baby does not drown, because its blood gets oxygen from the mother through the placenta’s action. Also, the baby swallows much of this fluid, from which it apparently gets some nourishment.

The amniotic sac renders another fine service to the baby. It provides the baby with an even temperature in its temporary “home.” Moreover, the baby is almost weightless in its liquid surroundings and is therefore free to kick, to move about and even to turn somersaults, all this strengthening its muscles for when it enters the world of atmosphere and will have to move about without the liquid support.