Amniotic fluid

This 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 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:

  • Protectection from outside injury by cushioning sudden blows or movements  
  • Allowing for freedom of fetal movement and permitting symmetrical musculoskeletal development  
  • Maintaining a relatively constant temperature for the environment surrounding the fetus, thus protecting the fetus from heat loss  
  • Permitting 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 abnormalities.

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

Removal of a sample of the fluid is called amniocentesis. This can provide information about the sex, state of health, and maturity of the fetus.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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