It’s been called "the window on pregnancy," and amniocentesis does allow doctors to "peek" inside an expectant mother’s body to check the health of her developing baby. The test involves analysis of a sample of the amniotic fluid, the liquid that surrounds the baby in the womb. Because the fluid contains fetal cells, the test can provide important clues about its health. Amniocentesis does have its limits, however: it can predict only some – not all – possible birth defects, and it carries risks that, though slight, need to be weighed.
What the test can tell you:
Amniocentesis is nearly 100 percent accurate in detecting chromosomal abnormalities, including Down’s syndrome, which causes mental retardation. It can also indicate certain genetic disorders in the fetus, such as a missing enzyme and any neural-tube defects, which impede the development of the fetus’s brain or spinal cord.
Amniocentesis is sometimes used late in a pregnancy to gauge the development of the baby’s lungs, in case an early delivery is necessary. It may also be used to monitor a fetus afflicted with RH disease, a blood disorder.
Thanks to amniocentesis, treatments for RH disease may be initiated while the baby is still in the womb. If there are other problems, plans for special procedures at the time of delivery may help the baby survive. The test results might also assist parents in making important decisions about the future of the pregnancy or enable them to be better prepared when the baby arrives.
Reviewing the risks:
Amniocentesis is usually performed between the 14th and the 18th week of pregnancy. This 15-minute procedure involves inserting an ultrathin needle through the mother’s abdomen into the uterus and withdrawing a sample of amniotic fluid. To reduce risks, the doctor is guided by ultrasound, an image showing the position of the needle within the uterus that is projected onto a computer screen. Some patients report mild cramping during or after the procedure. Analysis of the fluid takes two to four weeks.
Problems are rare, and the more experienced the doctor, the less likely they are to occur. Even so, the test does slightly increase the mother’s risk of infection, vaginal bleeding, and leakage of amniotic fluid. There is also a slight risk of miscarriage. Before the test, physicians usually counsel couples about the risks involved and about the implications of any of the possible results.
When to take the test:
To identify serious problems early, consider amniocentesis: if the mother is over 35 (the risk of Down’s syndrome rises as a woman ages); if certain birth defects are already present in the family, particularly in a sibling; and if there is an abnormal level of alpha-fetoprotein, produced by the fetus, in the mother’s blood. A low level may indicate Down’s syndrome; a high level, a neural-tube defect.