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Amniocentesis

Amniocentesis is a test that can be done during pregnancy to look for birth defects and genetic problems in the developing baby.

Alternative Names

Culture - amniotic fluid; Culture - amniotic cells; Alpha-fetoprotein - amniocentesis

How the test is performed

Amniocentesis removes a small amount of fluid from the sac around the baby in the womb (uterus). It is most often done in a doctor's office or medical center. You do not need to stay in the hospital.

You will have a pregnancy ultrasound first. This helps your health care provider see where the baby is in your womb.

Numbing medicine is then rubbed onto part of your belly. Sometimes, the medicine is given through a shot in the skin on the belly area.

The health care provider inserts a long, thin needle through your belly and into the womb. A small amount of fluid (about 4 teaspoons) is removed from the sac surrounding the baby.

The fluid is sent to a laboratory. Testing may include:

  • Genetic studies
  • Measurement of alpha-fetoprotein (AFP) levels

Results come back in about 2 weeks.

Amniocentesis is also used at times later in pregnancy to diagnose infection, to check to see if the baby’s lungs are developed and ready for delivery, and to remove excess fluid from around the baby when an abnormally high amount of fluid is made (polyhydramnios).

How to prepare for the test

Your bladder must be full for the ultrasound.

Before the test, blood may be taken to find out your blood type and Rh factor. You may get a shot of medicine called RhoGAM if you are Rh negative.

Why the test is performed

Amniocentesis is most often offered to women who are at increased risk for bearing a child with birth defects. This includes women who:

  • Will be 35 or older when they give birth
  • Had a screening test result that shows there may be a birth defect or other problem
  • Have had babies with birth defects in other pregnancies
  • Have a family history of genetic disorders

You may choose genetic counseling before the procedure. This will allow you to:

  • Learn about other prenatal tests
  • Make an informed decision regarding options for prenatal diagnosis

This test:

  • Is a diagnostic test, not a screening test
  • Is 99% accurate for diagnosing Down syndrome
  • Is usually done between 14 and 20 weeks

Amniocentesis can be used to diagnose many different gene and chromosome problems in the baby, including:

  • Anencephaly
  • Down syndrome
  • Rare, metabolic disorders that are passed down through families
  • Other genetic abnormalities, like trisomy 18

Normal Values

A normal result means:

  • No genetic or chromosome problems were seen in your baby
  • Bilirubin and alpha fetoprotein levels appear normal

Note: Even with normal results after an amniocentesis, a baby may still have other types of birth defects.

What abnormal results mean

An abnormal result may mean your baby has:

  • A gene or chromosome problem, such as Down syndrome or other abnormalities
  • Birth defects that involve the spine or brain, such as spina bifida

Talk to your doctor about the meaning of your specific test results.

  • How the condition or defect may be treated during or after the pregnancy
  • Special needs your child may have after birth
  • What other options you have regarding maintaining your pregnancy

What the risks are

Risks are minimal, but may include:

  • Infection or injury to the baby
  • Leaking of amniotic fluid
  • Vaginal bleeding

References

Simpson JL, Holzgreve W, Driscoll DA. Genetic counseling and genetic screening. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2012:chap 10.

Simpson JL, Richards DA, Otao L, Driscoll DA. Prenatal genetic diagnosis. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, Pa: Elsevier Churchill Livingstone;2012:chap 11.

Updated: 8/5/2013

Susan Storck, MD, FACOG, Chief, Eastside Department of Obstetrics and Gynecology, Group Health Cooperative of Puget Sound, Bellevue, Washington; Clinical Teaching Faculty, Department of Obstetrics and Gynecology, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.


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