The Fetal Environment: Amniotic Cavity, Fluid and Membranes

The amniotic cavity is a controlled environment, not simply a fluid-filled cushion. It surrounds the fetus and umbilical cord, permits movement for musculoskeletal development, limits external trauma, helps maintain a stable temperature, and provides the fluid needed for normal fetal breathing movements and lung development. The amnion is the inner membrane directly enclosing the fluid. The chorion lies outside it and contributes to the fetal membranes at the uterine interface. Together, the membranes form a barrier that retains fluid and separates the intrauterine environment from the maternal tissues. When the membranes rupture, the clinical problem is not only fluid loss; the protective barrier is also breached, increasing the risk of ascending infection. Amniotic fluid keeps the umbilical cord from being compressed between the fetus and uterine wall. That function explains a classic labour pattern: when fluid is markedly reduced, a contraction or fetal descent can press on the cord and produce abrupt, recurrent variable decelerations.