What fluid-filled sac surrounds the human embryo?

The fluid-filled sac that surrounds the human embryo is the amniotic sac.

The amniotic sac is made of thin but tough membranes and encloses the amniotic cavity, which contains amniotic fluid. This fluid cushions the developing embryo and later fetus, helps maintain a stable environment, and allows movement as the body develops. The inner membrane is the amnion; the outer chorion is associated with the placenta.

The amniotic sac begins forming very early in development, during the second week after fertilisation. It is one of four extraembryonic membranes, alongside the yolk sac, allantois, and chorion. These structures support development without becoming part of the embryo itself.

A common mix-up is to call the fluid itself the amniotic sac. The sac is the membrane-bound container; amniotic fluid is what fills it. Near birth, rupture of these membranes is commonly described as a pregnant person's “water breaking.”