The amniotic sac is a protective membrane that surrounds a developing fetus during pregnancy. It is filled with a clear fluid called amniotic fluid, which cushions the fetus and allows it to move freely. This fluid plays a crucial role in maintaining a stable temperature and providing a safe environment for the baby to grow. As the pregnancy progresses, the amniotic sac expands to accommodate the growing fetus. It also helps in the development of the lungs and digestive system by allowing the fetus to practice breathing and swallowing. The amniotic sac typically breaks during labor, a process often referred to as "water breaking."