Prenatal Test: Amniocentesis

What Is Amniocentesis?

In amniocentesis, doctors take a sample of the amniotic fluid that surrounds a baby to check for signs of problems such as chromosomal disorders, genetic problems, and neural tube defects.

Why Is Amniocentesis Done?

Doctors test things in the amniotic fluid sample, such as cells shed by the fetus that contain genetic information. Second-trimester amniocentesis (am-nee-oh-sen-TEE-sis) is most often done to check for:

• Down syndrome and other chromosome problems
• structural defects such as spina bifida
• inherited metabolic disorders like PKU (phenylketonuria)

Doctors might do this test later in pregnancy (in the third trimester) to check for infection and Rh incompatibility. It also can show if a baby's lungs are strong enough for the baby to breathe normally after birth. This can help doctors make decisions about inducing labor or trying to prevent early labor, depending on the situation. For instance, if a mother's water breaks early, the health care provider might try to delay delivering the baby to allow the lungs to mature.

Should I Have Amniocentesis?

Your health care provider may recommend this test if you:

• had an abnormal screening test for genetic or chromosomal disorders or neural tube defects
• are age 35 or older
• have a family history of genetic disorders (or a partner who does)
• have had a previous child with a birth defect or had a previous pregnancy with a chromosomal abnormality or neural tube defect

Amniocentesis can be very accurate — close to 100% — but only some disorders can be detected. The rate of miscarriage with this test is between 1 in 300 and 1 in 500. It also carries a low risk of uterine infection, which can also cause miscarriage, leakage of amniotic fluid, and injury to the fetus.

Talk to your doctor if this test is recommended for you to discuss the pros and cons of having it.

What Happens During Amniocentesis?

While watching with an ultrasound, the doctor inserts a needle through the mother's abdominal wall into the uterus to remove about 1 ounce of amniotic fluid. Some women report cramping when the needle enters the uterus or pressure while the doctor takes the sample.

The doctor may check the fetus' heartbeat after the test to make sure it's normal. Most doctors recommend rest for several hours afterward.

The cells in the fluid sample are grown in a special culture and then analyzed (the specific tests done on the fluid depend on personal and family medical history).

When Is Amniocentesis Done?

Amniocentesis is usually done between 15 and 20 weeks, but can be done later in pregnancy if needed.

When Are the Results Available?

Timing varies depending on what is being tested for, but results usually are available within 1 to 2 weeks. Tests of lung maturity are often available within a few hours.