Amniotic Sac

Definition - What does Amniotic Sac mean?

The amniotic sac is the lining that surrounds the fetus in the uterus and contains amniotic fluid. It is comprised of two membranes, called the amnion and the chorion. The amniotic sac and amniotic fluid protect the fetus throughout pregnancy. During an amniocentesis, the sac is punctured to obtain a sample of amniotic fluid for testing.

The amniotic sac is also called the bag of waters.

FertilitySmarts explains Amniotic Sac

The amniotic sac, and the amniotic fluid inside it start to form within days of conception. Together, they protect the fetus within the uterus throughout the pregnancy. This includes protection from extreme temperatures and from being bumped as the pregnant woman moves around. The amniotic fluid within the amniotic sac is breathed and swallowed by the fetus. After about week 10 of pregnancy, the fetus also passes small amounts of urine into the amniotic fluid. This fluid is important for lung and musculoskeletal development. The amniotic sac may be punctured in a procedure called an amniocentesis, in order to obtain a sample of amniotic fluid for genetic testing.

Prior to delivery of the fetus, the amniotic sac breaks open and releases the fluid contained inside it. This is sometimes referred to as a woman’s “water breaking.” If this happens before 37 weeks of completed pregnancy, it is called a premature rupture of membranes (PROM) and may result in preterm labor.

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