amnionsæk

The amnion is the innermost of the two fetal membranes that surround a developing amniote (including mammals, birds, and reptiles) during gestation. It is a thin, transparent sac that encloses the fetus and fills with amniotic fluid. This fluid plays a crucial role in protecting the fetus from physical shocks, regulating its temperature, and preventing it from adhering to the uterine wall. The amnion also contributes to the development of the fetal lungs by allowing them to practice breathing movements.

The amniotic sac, also known as the "water bag," is formed by the fusion of the amnion and the chorion, another fetal membrane. This sac ruptures, often referred to as "breaking of the water," typically towards the end of labor, signaling the imminent birth of the baby. The presence and integrity of the amniotic sac are vital for a healthy pregnancy. Any abnormalities in its formation or function can lead to complications. In some cases, amniotic fluid can be measured to assess fetal well-being. The amnion itself is composed of epithelial cells and is avascular, meaning it does not contain blood vessels.