fostermembraner

Fostermembraner, or fetal membranes, are the extraembryonic membranes that surround and protect the developing fetus during pregnancy. They form a protective sac filled with amniotic fluid and help maintain a stable intrauterine environment.

In humans, the two principal fetal membranes are the amnion and the chorion. The amnion is the inner membrane that directly lines the amniotic cavity and encloses the fetus with amniotic fluid. It originates from the cells of the inner cell mass and surrounding extraembryonic tissues and expands to create the amniotic sac. The chorion is the outer membrane, derived from trophoblasts and extraembryonic mesoderm, and it contributes to the placenta via chorionic villi. During development the amnion and chorion come into close apposition and ultimately form part of the placental membranes.

Together, these membranes provide mechanical protection, cushion the fetus, and help regulate the intrauterine environment. They participate in gas exchange and the transport of nutrients and waste across the placental interface and play a role in immune interactions between mother and fetus. The integrity of the membranes is important for pregnancy health.

Clinical relevance includes rupture of membranes, often termed water breaking, which signals the onset of labor or potential infection if prolonged. Premature rupture of membranes (PROM) is associated with risk of preterm birth. In obstetrics, assessment of membrane integrity and amniotic fluid status is routine to monitor fetal well-being.