The innermost of two transparent membranous sacs that completely surround the developing embryo and fetus (see figure below). It is partly responsible for producing the amniotic fluid that fills the sac.
Amniotic sac and related structures
See Amniocentesis, Amniotic fluid, Ascending genital tract infection, Chorion, Chorionic villus sampling, Dizygotic twins, Monozygotic twins, Placenta