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Amniocentesis

Definition

Amniocentesis is a test during pregnancy that removes a small amount of fluid from the sac around the baby to look for birth defects and chromosome problems.

Alternative Names

Culture - amniotic fluid; Culture - amniotic cells

How the test is performed

Amniocentesis is done on an outpatient basis. You do not need to stay in the hospital.

The doctor will find the exact location of the baby, usually by performing a pregnancy ultrasound.

The health care provider will clean an area of skin on the mother's belly area. A numbing medication (anesthetic) may be applied to the skin, or a local anesthetic may be injected into the skin.

The doctor inserts a long, thin needle through the abdomen and into the womb (uterus). A small amount of fluid is taken from the fluid-filled sac that surrounds the baby.

How to prepare for the test

Your bladder must be full for the ultrasound. There are no food or drink restrictions.

You may need to provide a blood sample to determine your blood type and Rh factor. You may get an injection of a medication called Rhogam if you are Rh negative.

You will need to sign a consent form before the test.

How the test will feel

If an anesthetic is used, you may feel a sharp, stinging sensation for a few seconds. When the needle enters the amniotic sac, you may feel a sharp pain lasting a few seconds.

Some women feel pressure in the lower abdomen when the fluid is pulled out. After the procedure, you may have some minor cramping.

Why the test is performed

The test can find chromosome problems such as:

• Anencephaly
• Down syndrome
• Rare, inherited metabolic disorders
• Spina bifida and other structure problems

Later on in a pregnancy, the test may be used to find problems such as:

• Infection
• Rh incompatibility

This test is also sometimes done later in pregnancy to determine whether the baby's lungs are developed if there is a condition that requires early delivery.

Normal Values

• No defects in the chromosomes
• No excess of alpha fetoprotein or bilirubin

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

Amniocentesis can be used to diagnose a large number of gene and chromosome problems in the baby. In addition, it can help:

• Determine how well the baby's lungs are developed
• Diagnose neural tube defects (such as spina bifida)
• Diagnose Rh incompatibility

DNA testing is available for many of the diseases that may be detected during amniocentesis. Ask your obstetrician or geneticist if you have a question about a specific disease.

What the risks are

Risks are minimal, but may include:

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

Special considerations

There may be alternatives to the amniocentesis. Discuss these other tests with your health care provider.

References

Simpson JL, Otaño L. Prenatal genetic diagnosis. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics:Normal and Problem Pregnancies. 5th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2007:chap 7.

Druzin ML, Smith JF Jr, Gabbe SG, Reed KL. Antepartum fetal evaluation. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 5th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2007:chap 11.

Cunnigham FG, Leveno KL, Bloom SL, et al . Prenatal diagnosis and fetal therapy. In: Cunnigham FG, Leveno KL, Bloom SL, et al, eds. Williams Obstetrics. 22nd ed. New York, NY; McGraw-Hill; 2005:chap 13.