Women who have an amniocentesis to rule out chromosomal abnormalities must weigh the peace of mind the test offers against the risk of pregnancy loss due to the procedure itself. Past studies have shown that amniocentesis -- done in weeks 15 to 18 of pregnancy, primarily in women over 35 -- leads to miscarriage in between 1 in 200 and 1 in 400 procedures. Many experts believe this risk is actually much lower today, though no one has studied the topic since these statistics were established some time ago.
"When we first studied amnios in the seventies, we didn't have ultrasound to see where the needle was going," explains Michael T. Mennuti, M.D., director of reproductive genetics and prenatal genetic diagnosis at the University of Pennsylvania. "Intuitively, I have to believe it's safer today." Years of testing also mean that more physicians have a greater degree of experience, he adds, though there are only anecdotal reports to back this up.
Other recent advances make amnios a little easier for expectant mothers. After amniocentesis, women usually must wait for the fetal cells taken from the amniotic fluid to be grown in a lab dish and tested for possible chromosomal abnormalities -- a process which takes about ten days to two weeks for results. With a new technology called FISH (fluorescence in-situ hybridization), however, a technician can screen for specific abnormalities (such as Down syndrome) in amniotic fluid cells at the molecular level, producing results in a few days and reducing the agonizing wait for expectant mothers, says Cynthia Curry, M.D., director of Genetic Medicine Central California, in Fresno, and professor of pediatrics at the University of California, San Francisco. The drawbacks: FISH is expensive, (about $250 to $300) and insurance companies may not cover the cost. Typically, it's used to screen for several specific genetic problems when an abnormality is strongly suspected or when time is a critical factor.
Even as amnios get safer -- and speedier -- new testing strategies could soon reduce the need for them. Recent research found that combining blood tests of markers associated with certain defects with a special kind of ultrasound assessment of the fetus offers a good idea of a baby's risk of problems like Down syndrome.
In the more distant future, women will be able to assess their fetus's health with a simple blood test done in early pregnancy. Researchers are working to develop a way to isolate fetal cells from a mother's bloodstream. "There's been progress on this front," Dr. Curry says. "But it's not ready for prime time yet."