The amniotic fluid that surrounds a fetus (unborn baby) plays a crucial role in normal development. This clear-colored liquid cushions and protects the baby. By the second trimester, the baby is able to breathe the fluid into the lungs and to swallow it. This promotes normal growth and development of the lungs and gastrointestinal system. Amniotic fluid allows the baby to move around, aiding development of muscles and bones.
The amniotic sac that contains the baby forms about 12 days after conception. Amniotic fluid immediately begins to fill the sac. In the early weeks of pregnancy, amniotic fluid consists mainly of water supplied by the mother. After about 20 weeks, fetal urine makes up most of the fluid. Amniotic fluid also contains nutrients, hormones and disease-fighting antibodies (1).
The amount of amniotic fluid increases until 28 to 32 weeks of pregnancy, when it measures about 1 quart. After that time, the level of fluid generally stays constant until the baby is full term (37 to 40 weeks), when the level begins to decline.
In some pregnancies, however, there may be too little or too much amniotic fluid.
- Having too little amniotic fluid is called oligohydramnios.
- Having too much amniotic fluid is called polyhydramnios.
Either condition can cause problems for mother and baby or be a sign of other problems. However, in most cases, the baby is born healthy.
How are oligohydramnios and polyhydramnios diagnosed?
An ultrasound examination can identify the amount of amniotic fluid. This procedure measures the depth of the fluid in four quadrants in the uterus. These amounts are then added up. This method results in a measurement of amniotic fluid called the amniotic fluid index (AFI). If the amniotic fluid depth measures less than 5 centimeters, the pregnant woman has oligohydramnios. If amniotic fluid levels add up to more than 24 centimeters, she has polyhydramnios (2).
How common is oligohydramnios?
About 4 percent of pregnant women have oligohydramnios (3). It can develop at any time during pregnancy, although it is most common in the last trimester. Some 12 percent of women whose pregnancies last about two weeks beyond their due dates (42 weeks gestation) develop oligohydramnios, because the level of amniotic fluid tends to decrease by that time in gestation (1, 2).
Does oligohydramnios pose risks to mother or baby?
The problems associated with oligohydramnios differ depending on the stage of the pregnancy. Oligohydramnios is more likely to have serious consequences if it occurs in the first half of pregnancy than if it occurs in the last trimester. These consequences include (2):
When oligohydramnios occurs in the second half of pregnancy, it may be associated with poor fetal growth. Near term, oligohydramnios may increase the risk of complications of labor and delivery, including compression of the umbilical cord. This can deprive the baby of oxygen, sometimes resulting in stillbirth. Women with oligohydramnios are more likely than unaffected women to need a cesarean section.
What causes too little amniotic fluid?
Many pregnant women develop oligohydramnios that has no identifiable cause. Known causes of oligohydramnios include:
- Premature rupture of the membranes (bag of waters that surrounds the baby).
- Birth defects, especially those involving the kidneys and urinary tract. Babies with these birth defects produce less urine, which makes up most of the amniotic fluid.
- Post-term pregnancy (a pregnancy that lasts two or more weeks past the due date). Levels of amniotic fluid tend to decrease after the baby reaches full term.
- Maternal health conditions, including pregestational diabetes mellitus, high blood pressure and systemic lupus erythematosus (SLE).
- Certain medications. A group of medications used to treat high blood pressure, called angiotensin-converting enzyme inhibitors (like captopril), can damage the fetal kidneys and cause severe oligohydramnios and fetal death. Women who have chronic high blood pressure should consult their provider before pregnancy to make sure that their blood pressure is under control and that any medications they take are safe during pregnancy.
How is oligohydramnios treated?
Women with otherwise normal pregnancies who develop oligohydramnios near term probably need no treatment, and their babies are likely to be born healthy (2). They do, however, require close surveillance. Their health care provider will probably recommend weekly (or more frequent) ultrasound examinations to see if the level of amniotic fluid is decreasing to a dangerous point. If the level of amniotic fluid becomes inadequate, the provider may recommend inducing labor early to help prevent complications during labor and delivery. Nearly half the cases of oligohydramnios resolve themselves without treatment (4).
Besides frequent ultrasound examinations to measure the level of amniotic fluid, providers may recommend tests of fetal well-being, such as the nonstress test, which measures fetal heart rate. If tests show that the baby is having difficulties, the provider may recommend early delivery to help prevent serious problems.
A fetus with poor growth whose mother has oligohydramnios is at high risk for complications, such as asphyxia (lack of oxygen), before and during birth. Mothers of these babies are monitored very closely. They sometimes need to be hospitalized.
If a woman has severe oligohydramnios near the time of delivery, her provider may suggest infusing salty water (saline solution) through the cervix into the uterus. This treatment, called amnioinfusion, may help reduce complications during labor and delivery and reduce the need for cesarean section. Some studies suggest that women with oligohydramnios can help increase their levels of amniotic fluid by drinking extra water (3). Also, many providers recommend decreasing physical activity or bed rest.
How common is polyhydramnios?
About 1 percent of pregnant women have too much amniotic fluid (2, 3). Most cases are minor and result from a gradual buildup of excess fluid in the second half of pregnancy. However, a small number of women have a rapid buildup of fluid occurring as early as 16 weeks of pregnancy that usually results in very early delivery (2).
What complications can polyhydramnios cause for mother and baby?
Women with minor polyhydramnios experience few symptoms. However, those who are more severely affected may have abdominal discomfort and breathing difficulties as a result of the uterus crowding the abdominal organs and lungs.
Polyhydramnios may increase the risk of pregnancy complications including (2):
- Premature delivery
- PROM
- Placental abruption (the placenta partially or completely peels away from the uterine wall before delivery)
- Stillbirth
- Postpartum hemorrhage (severe bleeding after delivery)
- Fetal malposition (the baby is not lying in a head-down position and may need to be delivered by cesarean section)
What causes polyhydramnios?
In about two-thirds of cases, the cause of polyhydramnios is unknown (2). About 20 percent of babies from pregnancies affected by polyhydramnios have a birth defect (3). Some of these birth defects contribute to polyhydramnios. The most common birth defects that cause polyhydramnios are those that hinder fetal swallowing, such as birth defects involving the gastrointestinal tract and central nervous system (2, 3). Normally, swallowing by the fetus, balanced by the production of fluid, maintains the fluid at a steady level.
Other less common causes of polyhydramnios include (3):
- Maternal-fetal blood incompatibilities (such as Rh disease)
- Twin-twin transfusion syndrome (TTTS)(a complication affecting identical-twin pregnancies, in which one fetus gets too much blood flow and the other too little because of connections between blood vessels in their shared placenta)
- Maternal diabetes
- Fetal infection, such as with parvovirus B19 (which in childhood commonly causes a mild illness called fifth disease)
How is polyhydramnios treated?
When a routine ultrasound examination shows evidence of polyhydramnios, a more detailed ultrasound examination is recommended to diagnose or rule out birth defects and TTTS. A woman's health care provider also may recommend amniocentesis (a small amount of amniotic fluid is removed through a needle inserted into the mother's abdomen to test for certain birth defects) and a blood test for diabetes.
In many cases, slight polyhydramnios goes away without treatment (2). In other cases, it may resolve when the problem causing it is corrected. For example, treating high blood sugar levels in women with diabetes or treating certain fetal heart rhythm disturbances (by medicating the mother) often reduces amniotic fluid levels.
Health care providers usually closely monitor women with polyhydramnios with weekly (or more frequent) ultrasound examinations to check amniotic fluid levels. Tests of fetal well-being also are recommended to check for signs of fetal difficulties. If the pregnant woman becomes too uncomfortable because of the extremely large volume of fluid, her provider may recommend treatment with indomethacin. This drug helps reduce fetal urine production and reduces amniotic fluid levels. Amniocentesis also can be used to drain off excess fluid. This procedure may have to be repeated a number of times.
If the tests show that mother and baby are healthy, a woman with slight polyhydramnios near term usually does not need any treatment.
Does discolored amniotic fluid indicate that there is a problem with the baby?
Normal amniotic fluid is clear or tinted yellow. Green- or brown-tinged fluid usually indicates that the baby has passed meconium (stool formed before birth that is normally released after birth in the first bowel movement). This can be a sign that the baby is under stress. Occasionally, a baby may breathe the fluid into the lungs, which may result in serious breathing problems (meconium aspiration syndrome) that require prompt treatment. When a provider sees meconium during labor, he may treat the mother with amnioinfusion with the hope that it may prevent these complications in the baby.
Pink-tinged fluid suggests recent bleeding; wine-colored amniotic fluid suggests bleeding in the past. These signs may be trivial, but tests may be suggested to find possible causes.
References
- Underwood, M.A., et al. State of the Art: Amniotic Fluid: Not Just Fetal Urine Anymore. Journal of Perinatology, volume 25, 2005, pages 341-348.
- Cunningham, F.G., et al. Disorders of Amniotic Fluid Volume, in: Williams Obstetrics 22nd Edition, New York, McGraw-Hill Medical Publishing Division, 2005, pages 525-534.
- Boyd, R.L. Polyhydramnios and Oligohydramnios. Emedicine, May 19, 2006.
- Ross, M.G., Brace, R.A., and NIH Workshop Participants. National Institute of Child Health and Development Conference Summary: Amniotic Fluid Biology—Basic and Clinical Aspects. Journal of Maternal-Fetal Medicine, volume 10, February 2001, pages 2-19.
July 2007
