Amniotic sac
A drawing of the amniotic sac from Gray's Anatomy.
The amniotic sac opened during afterbirth examination.

The amniotic sac (also bag of waters) is the sac in which the fetus develops in amniotes. It is a tough but thin transparent pair of membranes, which hold a developing embryo (and later fetus) until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the chorion, contains the amnion and is part of the placenta. Its wall is the amnion, the inner of the two fetal membranes. It encloses the amniotic cavity and the embryo. The amniotic cavity contains the amniotic fluid. On the outer side, the amniotic sac is connected to the yolk sac, to the allantois and, through the umbilical cord, to the placenta.^{[1][2][3][4][5][6][7][8]}

In mammals[link]

The amniotic sac and its filling provide a liquid that surrounds and cushions the fetus. It allows the fetus to move freely within the walls of the uterus. Buoyancy is also provided.

In humans[link]

If, after birth, the complete amniotic sac or big parts of the membrane remain coating the newborn, this is called a caul.

When seen in the light, the amniotic sac is shiny and very smooth, but tough.

Amniotomy[link]

An artificial rupture of membranes (AROM), also known as an amniotomy, may be performed by a midwife or obstetrician. This is usually performed using an amnihook or fingercot and is intended to induce or accelerate labour.

Diversity[link]

The presence of the amnion identifies humans and other mammals as amniotes, along with reptiles, dinosaurs, birds but neither amphibians nor fish.

See also[link]

• Chorion, the outer one of the two fetal membranes
• Chorioamnionitis

References[link]