Amnion
The thin, transparent inner membrane (the chorion is the outer layer) that constitutes the amniotic membranes. These membranes contain the amniotic fluid and form a protective layer for the baby insulating him/her from bacteria in the vagina. Rupture of this sac exposes the child to bacteria in the vagina and increases the risk of infection if the infant is not delivered within about 18 hours. Labor usually starts shortly after of before rupture of the amniotic membranes.