Pregnancy and Amniocentesis

During pregnancy, the fetus is surrounded by amniotic fluid, a substance much like water. Amniotic fluid contains live fetal skin cells that are normally shed during growth and other substances, such as alpha-fetoprotein (AFP). These substances provide important information about your baby's health before birth.

What Is Amniocentesis?

Amniocentesis is a prenatal test in which a small amount of amniotic fluid is removed from the sac surrounding the fetus and is tested. The sample of amniotic fluid (less than one ounce) is removed through a fine needle inserted into the uterus through the abdomen, under ultrasound guidance. The fluid is then sent to a laboratory for analysis. Different tests can be performed on a sample of amniotic fluid, depending on the genetic risk and indication for the test.

Why Is an Amniocentesis Performed?

Amniocentesis is performed to look for certain types of birth defects, such as Down syndrome, a chromosomal abnormality.

Because amniocentesis presents a small risk for both the mother and her baby, the prenatal test is generally offered to women who have a significant risk for genetic diseases, including those who:

• Have an abnormal ultrasound.
• Have a family history of certain birth defects.
• Have previously had a child or pregnancy with a birth defect.
• Will be 35 or older at the time of delivery.

Amniocentesis does not detect all birth defects, but it can be used to detect the following conditions if the parents have a significant genetic risk:

• Down syndrome
• Sickle cell disease
• Cystic fibrosis
• Muscular dystrophy
• Tay-Sachs and similar diseases

Amniocentesis can also detect certain neural tube defects (diseases where the brain and spinal column don't develop properly) such as spina bifida and anencephaly.

Because ultrasound is performed at the time of amniocentesis, it may detect birth defects that are not detected by amniocentesis (such as cleft palate, cleft lip, club foot, or heart defects). There are some birth defects, however, that will not be detected by either amniocentesis or ultrasound.

If you are having an amniocentesis, you may ask to find out the baby's sex; amniocentesis is the most accurate way to determine the baby's gender before birth.

An amniocentesis can also be done during the third trimester of the pregnancy to determine if the baby's lungs are mature enough for delivery, in cases where an early delivery may be warranted. Another indication could be to evaluate for infection in the amniotic fluid in certain situations.

When Is Amniocentesis Performed?

If your doctor has recommended an amniocentesis, the procedure is usually scheduled between the 15th and 18th week of pregnancy.

How Accurate Is Amniocentesis?

The accuracy of amniocentesis is about 99.4%.

Amniocentesis may occasionally be unsuccessful due to technical problems such as being unable to collect an adequate amount of amniotic fluid or failure of the collected cells to grow when cultured.