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 approximately 34 weeks of 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 replacing it through "exhalation" and urination.

Amniotic fluid accomplishes numerous functions for the fetus, including:

• Protects from outside injury by cushioning sudden blows or movements
• Allows for freedom of fetal movement and permits symmetrical musculoskeletal development
• Maintains a relatively constant temperature for the environment surrounding the fetus, thus protecting the fetus from heat loss
• Permits proper lung development

An excessive amount of amniotic fluid is called polyhydramnios. This condition may accompany multiple pregnancy (twins or triplets), congenital anomalies, or gestational diabetes.

An abnormally small amount of amniotic fluid is known as oligohydramnios. This condition may accompany postdates pregnancies, ruptured membranes, placental dysfunction, or fetal anomalies.

Abnormal amounts of amniotic fluid may trigger additional surveillance of the pregnancy.

The fluid provides a great deal of knowledge concerning the sex, state of health, and maturity of the fetus. Removal of a sample of the fluid is called amniocentesis.