Amniotic fluid is the fluid that surrounds the fetus in the uterus. The fluid and fetus are contained in membranes called the amniotic sac. There may be too much or too little amniotic fluid.

Too much amniotic fluid (polyhydramnios or hydramnios) stretches the uterus and puts pressure on the diaphragm of pregnant women. This complication can lead to severe breathing problems for women or to labor that begins before 37 weeks of pregnancy (preterm labor).

Too much fluid may accumulate because of the following:

• Diabetes in the pregnant woman
• More than one fetus (multiple births)
• Rh antibodies to the fetus's blood produced by the pregnant woman (Rh incompatibility)
• Birth defects in the fetus, especially a blocked esophagus or defects of the brain and spinal cord (such as spina bifida)

However, about half the time, the cause is unknown.

Too little amniotic fluid (oligohydramnios) also can cause problems. If the amount of fluid is greatly reduced, the fetus's lungs may be immature and the fetus may be compressed, resulting in deformities. This combination of conditions is called Potter's syndrome.

There tends to be too little amniotic fluid in the following situations:

• The fetus has birth defects in the urinary tract.
• The fetus has not grown as much as expected.
• The fetus has died.
• The fetus has a chromosomal abnormality.
• The placenta is not functioning normally (as a result, the fetus may not grow as much as expected).
• The pregnancy has lasted too long (42 weeks or more).

Taking certain drugs such as angiotensin-converting enzyme (ACE) inhibitors (including enalapril or captopril) during the 2nd and 3rd trimesters can result in too little amniotic fluid. These drugs are usually avoided during pregnancy but sometimes may be needed to treat severe heart failure or high blood pressure. Taking nonsteroidal anti-inflammatory drugs (NSAIDs, such as aspirin or ibuprofen) late in pregnancy also can reduce the amount of amniotic fluid.

Doctors may suspect too much or too little amniotic fluid when the uterus is too large or too small for the length of the pregnancy. Sometimes the problem is incidentally detected during ultrasonography.

Last full review/revision December 2008 by Edmund F. Funai, MD