The amniotic sac is a bag of fluid inside a woman’s womb (uterus) where the unborn baby develops and grows. It’s sometimes called the “membranes” because the sac is made of 2 membranes called the amnion and the chorion.
The amniotic sac develops as a thin echogenic structure surrounding the embryo. The amniotic sac appears following the appearance of the yolk sac and just before the appearance of the embryo. Whereas the gestational sac shows variations in size and shape, the growth of the amniotic sac is closely related to that of the embryo between 6 and 10 weeks.