The sac that contains the amniotic fluid (or amnii) and the developing embryo/fetus in pregnant mammals, reptiles, and birds.

The sac is composed of two layers. The thin tough inner membrane of the amniotic sac is called the amnion and is primarily protective in function. The outer layer is called the chorion. The chorion plays an important role in carrying nutrients to the developing embryo.

Also known as the bag of waters.