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.

The health care provider will find the exact location of the baby, usually by ultrasound (see pregnancy ultrasound).

The skin of the abdomen is scrubbed. A numbing medication (anesthetic) may be applied to the skin, or a local anesthetic may be injected into the skin.

A long, thin needle is inserted through the abdomen and into the uterus. A small amount of fluid is taken from the fluid-filled sac that surrounds the fetus.

This test is done on the same day -- you will not need to stay overnight in the hospital. You must sign a consent form. 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.

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.

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

Late in the pregnancy, this test can determine whether the baby's lungs are developed.

• 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.

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

• Lung maturity
• Neural tube defects (such as spina bifida)
• Rh incompatibility

DNA testing is available for many diseases. New diseases are being added to this list as genetic research advances. Ask your obstetrician or geneticist if you have a question about a specific disease.

Risks are minimal, but may include:

• Infection or injury to the baby
• Miscarriage

This test is typically done when a problem is suspected, so the benefits outweigh the risks.

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