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. The fluid, amniotic sac, and/or placenta may become infected (see Intra-Amniotic Infection).

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 early—before 37 weeks of pregnancy (preterm labor).

Too much fluid may accumulate because of the following:

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

Too little amniotic fluid (oligohydramnios) can also cause problems. If the amount of fluid is greatly reduced, the fetus may be compressed, resulting in deformities. When oligohydramnios is present, the lungs may not mature normally. This combination of immature lungs and deformities is called Potter syndrome.

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

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. However, rarely, they are used to treat severe heart failure. Taking nonsteroidal anti-inflammatory drugs (NSAIDs, such as aspirin or ibuprofen) late in pregnancy can also 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.