Amniocentesis

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 Rho(D) Immune Globulin (RhoGAM and other brands) if you are Rh negative.

Why the Test is Performed

Amniocentesis is usually offered to women who are at increased risk of having a child with birth defects. This includes women who:

• Will be 35 or older when they give birth
• Had a screening test 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

Genetic counseling is recommended before the procedure. This will allow you to:

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

This test:

• Is a diagnostic test, not a screening test
• Is greater than 99% accurate for diagnosing Down syndrome
• Is most often done between 15 and 20 weeks

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

• Anencephaly (when the baby is missing a large portion of the brain)
• Down syndrome
• Rare metabolic disorders that are passed down through families
• Other genetic problems, like trisomy 18

Normal Results

A normal result means:

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

Note: Even with normal amniocentesis results, a baby may still have genetic or 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 
• Birth defects that involve the spine or brain, such as spina bifida

Talk to your health care provider about the meaning of your specific test results. Ask your provider:

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

Risks

Risks are minimal, but may include:

• Infection or injury to the baby
• Miscarriage
• 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.