The amniotic membrane is a thin, flexible sac that surrounds and protects a developing fetus during pregnancy. It is composed mainly of two layers of cells, with a thick layer of fluid between them, which helps to cushion the baby from external shocks and maintain optimal temperature and humidity levels within the womb. The amniotic membrane also plays an important role in facilitating fetal lung development by allowing the exchange of gases between the fetus and the mother's bloodstream through the umbilical cord.