Amniocentesis is a test to look at the amniotic fluid that surrounds your baby. This fluid has cells and other substances in it that can give clues about your baby's health.

During the test, your doctor puts a needle through your belly into your uterus. He or she takes out about 2 Tbsp (30 mL) of amniotic fluid to send to a lab. The cells in the fluid are checked to see if your baby has any signs of a serious health problem.

Amniocentesis is usually done between weeks 15 and 20 of a woman's pregnancy to check for certain health problems. It's not a routine test. But your doctor may recommend it if a fetal ultrasound and blood tests suggest that your chances of having a baby with a genetic disorder or a birth defect are higher than average. Or you may already know that your chances are higher because of your age and family history.

Some women don't want to wait until their second trimester to see if their baby might have a problem. They might decide to have a chorionic villus sampling (CVS) test instead of amniocentesis. CVS can be done between 10 and 12 weeks of pregnancy to look for Down syndrome and other serious health problems. But it can't find certain problems, such as birth defects of the heart, stomach, intestines, or brain and spine (neural tube defect).

Amniocentesis may also be done during the third trimester of pregnancy to see if your baby's lungs are fully formed. This test may be recommended if you need to deliver early because of a problem with your pregnancy. The test may also be done to check the amniotic fluid for infection.

Amniocentesis can tell you if your baby is at risk for:

• Genetic disorders or birth defects, such as Down syndrome and neural tube defects.
• Certain diseases that run in families, such as cystic fibrosis, sickle cell disease, and Huntington's disease.

When amniocentesis is done during your third trimester, it can tell you if your baby's lungs are fully formed. The test can also show whether there is an infection in the amniotic fluid.

Even if the results from your amniocentesis are normal, it doesn't guarantee that your baby will be born healthy. For example, this test can't find many common birth defects, such as cleft lip, cleft palate, and heart problems. No single test can find all problems.

This test can tell you if your baby may be born with a serious health problem. Many parents are not prepared to care for a baby who is sick or has a birth defect. Information that you get from this test can help you and your partner plan for the future.

If the test finds that your baby has a genetic disorder or a birth defect, you and your partner may be faced with a tough decision about whether to continue the pregnancy. You may find it helpful to talk with your doctor and a genetic counselor. They can help you understand your baby's health problem and what to expect when he or she is born.

Results from the test can also help you decide where to have your baby. If your baby will need surgery or special care, you can plan to have your baby in a hospital that has special services for newborns, such as a neonatal intensive care unit.

Amniocentesis is usually very safe. But the test does have some risks. You'll have to weigh the risks against the benefits of knowing if something might be wrong with your baby.

There is a chance that the test may cause you to have a miscarriage. This means that you could lose your baby after you have the test. But when the test is done by highly trained doctors, the risk of having a miscarriage is small.

• In one study of women who had amniocentesis performed by a highly trained doctor, about 1 out of 400 women had a miscarriage after the test. This means that 399 did not.¹
• Another study showed that 2 to 4 out of 400 women had a miscarriage after the test. This means that 396 to 398 did not. This greater risk may be more likely in medical centers whose doctors have less experience in doing the test.²

Other risks include:

• Infection. There is a very small chance that the amniotic fluid may get infected.³
• Injury. There is a chance that your baby may be poked by the needle during the test. But this is rare when ultrasound is used to guide the needle. The most common injury is a tear in the placenta. This usually heals without a problem.
• Bleeding. There is a small chance that you could bleed during the test. If this happens, your blood may mix with your baby's blood. This is only a problem if your blood is Rh-negative and your baby's is Rh-positive, because you could have an immune system response called Rh sensitization. If you're at risk for Rh sensitization, you'll be given a vaccine to prevent it.
• Foot defect. There is an increased risk that your baby may be born with a clubfoot when amniocentesis is done before 15 weeks of pregnancy.

All patient decisions are personal. You might think about:

• Your chance of passing on a family disease to your baby.
• Your age. As you get older, you have a greater chance of having a baby with a birth defect. Many doctors use 35 and older as the age for higher risk.
• Your need to know about any problems with your baby.
• What you might do if the test shows a problem.
• Whether you can afford to pay for the test. Amniocentesis can cost a lot. Most insurance companies will cover the cost of the test if you have certain risk factors that may increase your baby's chance of having a serious health problem. A risk factor is something—such as your age or family history—that raises your risk of having a certain health problem.

Your doctor may advise you to have amniocentesis if:

• Other tests suggest that your chance of having a baby with a genetic disorder or birth defect is higher than average.
• You or your partner has a family history of birth defects.
• You or your partner carries an abnormal gene that is known to cause a certain disease.
• You want to know if your baby has a serious health problem so you can decide early whether you want to continue your pregnancy or make plans to care for a sick child.