Definition of Amniocentesis:

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.

How the test is performed:

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.

How to prepare for the test:

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.

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

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