What is amniocentesis?

This is a prenatal test that examines cells shed by the fetus into the surrounding amniotic fluid. It is a reliable indicator of chromosomal abnormalities such as Down's syndrome (a chromosomal disorder that results in mental retardation and physical abnormalities), genetic disorders such as cystic fibrosis, and neural tube defects such as spina bifida.

Why would I need this test?

This test is not necessary for every pregnancy. However, it may be appropriate for you if you are 35 years or older on your due date and/or if you have a history of birth defects in your immediate family. Amniocentesis may also be done as a follow-up procedure if the result of an alpha-fetoprotein screening (AFP) test is positive, or to determine whether the lungs are mature enough for an early birth.

Just because your doctor recommended an amniocentesis doesn't mean that it is required. Weigh the potential risks that the test poses with the benefits of knowing the results. Discuss any questions and concerns with your doctor or a genetics counselor before undergoing the test.

When is this test done?

Amniocentesis is typically done at 16 to 18 weeks of pregnancy, when there are enough cells in the amniotic fluid to be tested.

How shall I prepare for amniocentesis?

No special preparation is needed before the test. For two to three days after the test, you should take it easy and drink plenty of fluids.

What happens during this test?

Your skin is prepared with an antiseptic swab; then an anesthetic to numb the area may be injected, which may cause an initial stinging sensation. An ultrasound is used to determine the position of the fetus and the location of the placenta. Using the ultrasound images for guidance throughout, the doctor then inserts a long, thin needle through the mother's abdomen to extract amniotic fluid from the womb.

Once the amniotic fluid is collected, the cells are cultured and stimulated to grow for one to two weeks. After that, the cell chromosomes can be examined. For this reason, test results are not available for several weeks after the test is performed.

How long does it take?

The procedure takes about 10 minutes.

What are the risks?

Amniocentesis is usually safe. However it carries a slight risk of miscarriage, so it is important to ask your health care provider for information to help you decide if you want this test.

You may experience mild symptoms such as cramping and a small amount of spotting after an amniocentesis. Your doctor will give you specific instructions regarding which symptoms to report. Generally speaking, notify your doctor if you experience vaginal bleeding, contractions or abdominal cramping that is strong, leakage of amniotic fluid or fever. Any of these symptoms could be associated with a complication from this procedure.

What are normal results?

Amniocentesis is about 99 percent accurate for detecting Down's syndrome. It is also better than 90 percent accurate in detecting a host of other genetic defects. Remember, not every genetic test is performed on amniotic fluid. Talk with your doctor to determine exactly which tests will be performed on your amniotic fluid.