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Amniocentesis: Answers to common questions
From MayoClinic.com
Special to CNN.com

Prenatal testing can provide valuable information about your baby's health. But the decision to pursue invasive diagnostic tests is serious. If you're considering amniocentesis, here's what you need to know.

What is amniocentesis?

Amniocentesis is a prenatal test used to identify various genetic problems or test a baby's lung maturity. Using a thin needle inserted into your uterus, your health care provider withdraws a sample of the amniotic fluid that surrounds and protects the baby. This fluid contains fetal cells and various chemicals produced by the baby.

With genetic amniocentesis, the chromosomes and genes in these cells or the chemicals in the amniotic fluid can be tested for certain abnormalities, such as Down syndrome and spina bifida. With maturity amniocentesis, the amniotic fluid is tested to determine whether a baby's lungs are mature enough for birth.

Who might need amniocentesis?

Genetic amniocentesis is typically offered to women age 35 or older, whose babies have an increased risk of chromosomal abnormalities. Other reasons to consider genetic amniocentesis include:

  • A chromosomal abnormality or neural tube defect in a previous pregnancy
  • Abnormal results from a prenatal screening test, such as first trimester screening or the quad marker screen
  • A family history of central nervous system defects, Down syndrome or other genetic disorders

Maturity amniocentesis — to determine whether the baby's lungs are ready for birth — is only needed if early delivery would be best for the mother.

When is amniocentesis done?

Genetic amniocentesis is usually done after the 15th week of pregnancy. At this point, the two layers of fetal membranes have fused enough to safely withdraw a sample of amniotic fluid for evaluation. Maturity amniocentesis is most common after the 36th week of pregnancy.

What happens during amniocentesis?

Amniocentesis can be done in the doctor's office. Your health care provider will use ultrasound to determine the baby's exact location in your uterus. Then your abdomen will be cleaned with an antiseptic. Guided by ultrasound, your health care provider will insert a thin, hollow needle through your abdominal wall and into your uterus. About 2 to 4 teaspoons of amniotic fluid will be withdrawn into a syringe, and the needle will be removed.

You'll notice a stinging sensation when the needle enters your skin. Many women feel mild cramps as the needle passes through the uterus. The baby quickly replaces the small amount of fluid that's removed.

When are test results available?

The sample of amniotic fluid is analyzed in a lab. For genetic analysis, some results may be available within a few days. Traditional chromosomal assessment takes up to 14 days — long enough for the fetal cells to multiply until there are enough to be tested. Results of maturity amniocentesis are available within hours.

What can amniocentesis reveal?

Genetic amniocentesis can identify chromosomal abnormalities and certain genetic problems, including Down syndrome and spina bifida. Many other conditions — including cystic fibrosis, hemophilia and sickle cell disease — can be diagnosed using amniocentesis as well, but only if there is a specific reason to test for these conditions.

Less often, amniocentesis is used to diagnose uterine infections or Rh incompatibility — an uncommon condition in which a mother's immune system produces antibodies against a specific protein on the surface of the baby's blood cells.

By analyzing the types of compounds produced in the baby's lungs, maturity amniocentesis can determine if the baby is ready to breathe air.

How accurate are the results?

Genetic amniocentesis accurately identifies certain genetic disorders, such as Down syndrome. But amniocentesis can't identify all birth defects, including heart defects, clubfoot or cleft lip and palate. When maturity amniocentesis indicates lung maturity, the risk of a false reading is less than 1 percent.

What are the risks?

Amniocentesis carries various risks, including:

  • Miscarriage. Amniocentesis carries a slight risk of miscarriage, often due to rupture of the amniotic sac. The risk of miscarriage is highest when the procedure is done early in pregnancy, before the two layers of fetal membranes have sealed. By the second trimester, however, the risk of miscarriage drops. In fact, in a 2006 study, researchers found no notable difference in miscarriage rates for women who had amniocentesis during the second trimester and women who didn't.

    When amniocentesis is used later in pregnancy to assess a baby's lung maturity, rupture of the amniotic sac causes little concern because safe delivery is nearly always possible at that point.

  • Other complications. Cramping is possible after amniocentesis, and up to 3 percent of women experience light vaginal bleeding. Up to 1 percent of women leak amniotic fluid through the vagina, which increases the risk of orthopedic problems in early infancy and childhood. Infection is rare. If you develop a fever following an amniocentesis, contact your health care provider.
  • Rh sensitization. Rarely, amniocentesis may cause the baby's blood cells to enter the mother's bloodstream. If you have Rh negative blood and your baby has Rh positive blood, you'll be given a drug called Rh immunoglobulin after amniocentesis to prevent you from producing antibodies against your baby's blood cells.
  • Needle injury. During amniocentesis, the baby may move an arm or leg into the path of the needle. Serious needle injuries are rare.

Genetic amniocentesis is offered when the test results may have a significant impact on the management of the pregnancy. Often, that value can only be assessed by you and your partner. Ultimately, the decision to have genetic amniocentesis is up to you. Your health care provider or genetic counselor can help you weigh all the factors in the decision.

Maturity amniocentesis is often suggested when early delivery would be best for the mother. With minimal risks, maturity amniocentesis can offer assurance that the baby is ready for birth.

What happens after genetic amniocentesis?

Most test results are normal, which can ease anxiety about your baby's health. But if amniocentesis indicates that your baby has a chromosomal problem or a hereditary disorder that can't be treated, you may be faced with wrenching decisions — such as whether to continue the pregnancy. Seek support from your health care team, your loved ones and other close contacts during this difficult time.

  • Down syndrome: New guidelines for prenatal screening
  • Prenatal testing: Common screening and diagnostic tests
  • Prenatal testing: Who should consider it?
  • Chorionic villus sampling: Answers to common questions
  • Slide show: Fetal ultrasound
  • Fetal ultrasound: What can it tell you?
  • Video: Fetal ultrasound
  • November 21, 2006

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