What Is the Function of the Amniotic Sac?

The amniotic sac is a composed of two thin transparent membranes, the amnion and the chorion .The chorion makes up the outer layer and the amnion comprises the inner layer that holds the amniotic fluid. This fluid surrounds and cushions an unborn baby until birth, providing protection and regulating body temperature.

Time Frame

The amniotic sac begins to fill with fluid shortly after conception; sometime in the first two weeks of pregnancy. The amount of fluid increases as the baby grows and then decreases slightly at about 38 weeks gestation. Immediately prior or during labor the sac ruptures and the fluid is drained through the vagina.


Amniotic fluid is a pale yellow fluid that serves to cushion the unborn baby from jolts and bumps. It provides a stable temperature and assists in maintaining a consistent body temperature for the unborn child. The fluid allows the baby to move freely and exercise growing muscles to develop a symmetrical musculoskeletal system. It allows for lung development as the baby begins to breathe amniotic fluid to strengthen his lungs and swallow fluid to develop the gastrointestinal tract. The kidneys develop as the baby urinates in the fluid.


Premature rupture of the amniotic sac, often referred to as water breaking, increases the risk of infection for the unborn baby and requires medical observation. Labor is often more difficult and may be prolonged in women whose amniotic sac has ruptured prior to the onset of labor. If the amniotic sac has not ruptured, your doctor or midwife may break the sac to initiate labor.


Inadequate amounts of amniotic fluid, called oligohydramnios, may be an indication of ruptured membranes, dysfunction of the placenta or abnormalities with the fetus. Excessive amounts of amniotic fluid called, polyhydramnios, may indicate gestational diabetes or congenital defects.


If defects are suspected or congenital disease is indicated, your doctor may elect to perform amniocentesis to examine the amniotic fluid for chromosome defects or other indications of birth defects. This is performed by inserting a long thin needle into the belly, puncturing the amniotic sac, and drawing about 2 tablespoons of animistic fluid. This is generally done at 16 weeks if conditions such as Spina Bifida are suspected. Amniocentesis comes with a 1in 400 risk of miscarriage, with some estimates as high as 4 in 400.


The belief that bathing or swimming while pregnant will cause water to enter the uterus and introduce bacteria to the amniotic fluid is totally unfounded. Water does not enter the vagina and travel to the uterus when bathing or swimming.