Amniocentesis (called "amnio" for short) is usually performed between the 15th and 18th weeks of pregnancy.
Who needs to take the test, and why?
Amniocentesis (or "amnio") is currently the most accurate test available for detecting a variety of birth defects and genetic diseases. All pregnant women age 35 and over are offered amnio, because they face an increased risk of having a child with chromosomal abnormality such as Down syndrome. Your doctor may also recommend the test if you have a family history of genetic disease or chromosomal abnormality, or if your ultrasound, alpha-fetoprotein levels, or multiple marker screening tests have yielded suspicious results. Some women who don't meet any of the criteria for the test choose to have one for their own peace of mind. Amnio also reveals the sex of the baby, however, it is never given just for this reason (and your doctor can keep the sex a secret if you'd rather be surprised).
Amniocentesis is generally considered safe. It can lead to miscarriage in about 1 in 200 to 1 in 400 women. In determining whether to offer you an amnio test, your practitioner weighs this miscarriage risk against the risk for a woman your age of having a child with a chromosomal abnormality.
Not all women who are offered amnio opt to undergo the test. If you're certain you would never terminate a pregnancy for any reason, or wouldn't want to know in advance that your baby might have special needs, you should discuss your feelings with your healthcare provider. He or she can help you decide if having amniocentesis would be worthwhile.
The first step of the test is an ultrasound, which is used to measure the baby and ensure that you're at least 14 weeks pregnant (taking the test earlier than that can be harmful to the developing baby). Then an antibacterial iodine solution is rubbed onto your abdomen. Next, using the ultrasound transducer to get an image of the baby's position, the doctor inserts a long, narrow needle through your abdominal wall and into your uterus. Navigating carefully via the image on the ultrasound monitor, the doctor withdraws about an ounce of amniotic fluid, the liquid that surrounds the baby and contains some of its cells. The needle is then withdrawn, and the sample is sent to the lab for analysis. In the lab, fetal cells are grown and cultured and the chromosomes are analyzed for abnormalities. Couples who are carriers of genetic diseases such as Tay-Sachs or cystic fibrosis will receive a more targeted analysis of the amniotic sample.
The procedure takes about a minute and shouldn't hurt much more than having blood drawn. Afterward, you'll be advised to take it easy for a day or two. Side effects may include mild cramps, back pain, or discomfort around the needle site. Call your doctor immediately if you develop any bleeding, fluid leakage, or fever.
When are test results available, and how are they interpreted?
Probably the hardest part of having an amniocentesis is waiting for the results, which can take up to two weeks. Amnio is 99 percent accurate for detecting Down syndrome and about 90 percent accurate for detecting other chromosomal abnormalities and spina bifida. If the lab's results indicate a problem, you will receive counseling from your practitioner and a genetic specialist. Such cases can be exceedingly difficult for expectant parents, who may face agonizing decisions at a relatively late stage of pregnancy. For this reason, many healthcare providers are turning to chorionic villus sampling (CVS), a similar test that can be performed in the first trimester. And medical researchers are busy developing diagnostic blood tests and advanced ultrasounds that deliver earlier results and may one day replace amnio altogether.
All content here, including advice from doctors and other health professionals, should be considered as opinion only. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.