Amniocentesis

How the Test is Performed

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

You will probably have a pregnancy ultrasound first. This helps your health care provider find out exactly 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, where:

• Genetic studies are done.
• Levels of alpha-fetoprotein are measured.
• Other tests may be done.

Results are usually back within 2 weeks.

How to Prepare for the Test

Your bladder must be full for the ultrasound.

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

How the Test Will Feel

Amniocentesis is most often offered to women 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 indicates a possible birth defect or other problem.
• Have had babies with birth defects in previous pregnancies.
• Have a family history of genetic disorders

Genetic counseling is an option prior to the procedure. This will allow you to:

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

This test can also be done to help:

• Determine how well your baby's lungs are developed late in the pregnancy
• Detect a blood type mismatch between the mom and the baby (Rh incompatibility)
• Diagnose an infection in the baby

Why the Test is Performed

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

It can also help:

• Determine how well the baby's lungs are developed
• Detect spina bifida and other neural tube defects
• Detect Rh incompatibility
• Diagnose an infection in the baby

Normal Results

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, your 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 and many others
• 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 either during or after the pregnancy
• Special needs your child my have after birth
• What other options you have regarding maintaining your pregnancy

Risks

Risks are minimal, but may include:

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

Alternative Names

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

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.

Update Date: 8/7/2012

Updated by: A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, and David R. Eltz. Previously reviewed by Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine; 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 (9/13/2011)