A fluid-filled structure surrounding an embryo that develops within the uterine cavity early in pregnancy.

A fluid filled sac that develops within the cavity of the womb early in pregnancy.

A fluid-filled bag of membranes in which the embryo forms during pregnancy. Visible on transvaginal ultrasound from about 5 weeks from the last menstrual period. Technically, the amniotic cavity (and later in pregnancy able to be sampled with amniocentesis).

The gestational sac is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy (IUP) exists, until the embryo is identified. On ultrasound, it is an anechoic (dark) space surrounded by a hyperchoic (white) rim. It is spherical in shape, and usually located in the upper uterine fundus.