What does it test for?
If your blood test was abnormal and the ultrasound didn't uncover the cause, your doctor may recommend an amniocentesis to test for birth defects. Also, because the risk of having a baby with birth defects rises as you age, your healthcare provider may want to do an amniocentesis as a precautionary measure between 15 and 18 weeks if you're over 35. Amniocentesis is accurate in diagnosing chromosomal birth defects (such as Down syndrome), a wide variety of genetic birth defects (such as muscular dystrophy and cystic fibrosis), and certain malformations (such as neural tube defects or spina bifida).
How is it performed?
One or two tablespoons of amniotic fluid are extracted from your abdomen with a thin needle. Some women say amnio doesn't hurt at all; others feel cramping when the needle enters the uterus or pressure during the short time the fluid is being withdrawn. One to 2 percent of women experience cramping, leaking amniotic fluid, or spotting after the procedure (which you should report to your doctor). Most physicians recommend that you rest for several hours after the test.
Is it dangerous?
Amniocentesis does pose a small risk of miscarriage, so it's your choice whether or not to go ahead with the procedure. According to the Centers for Disease Control and Prevention (CDC), the rate of miscarriage after amniocentesis is between one in 200 and one in 400. The risk is three times higher if it's performed in the first trimester. It also carries an extremely low risk of uterine infection (less than one in 1,000), which may lead to miscarriage.
What if the results are abnormal?
The results can take two to three weeks, which is a long time to wait if you're nervous. However, the test provides you an opportunity to prepare ahead of time if there are birth defects -- and in some cases, problems can be treated while the baby is still in the womb.