Amniotic fluid is a clear, slightly yellowish liquid that surrounds the unborn baby (fetus) during pregnancy. It is contained in the amniotic sac.

The fetus floats in the amniotic fluid. During pregnancy the amniotic fluid increases in volume as the fetus grows. Amniotic fluid volume is greatest at about 34 weeks into the pregnancy (gestation), when it averages 800 ml. Approximately 600 ml of amniotic fluid surrounds the baby at full term (40 weeks gestation). This fluid is constantly circulated by the baby swallowing and "inhaling" existing fluid and then "exhaling" and urinating out the fluid.

Amniotic fluid performs many functions for the fetus, including:

• Allowing the fetus freedom to move and enabling the skeleton to develop properly
• Allowing the lungs to develop properly
• Maintaining a relatively constant temperature around the fetus, thus protecting the fetus from heat loss
• Protecting the fetus from outside injury by cushioning sudden blows or movements

An excessive amount of amniotic fluid is called polyhydramnios. This condition can occur with multiple pregnancy (twins or triplets), congenital anomalies (problems that exist when the baby is born), or gestational diabetes.

An abnormally small amount of amniotic fluid is known as oligohydramnios. This condition may occur with late pregnancies, ruptured membranes, placental dysfunction, or fetal abnormalities.

Abnormal amounts of amniotic fluid may cause the health care provider to watch the pregnancy more carefully. Removing a sample of the fluid, called amniocentesis, can provide information about the sex, health, and maturity of the fetus.