Women's Health: Amnio risk is lower than you think

Amniocentesis – a prenatal test for genetic abnormalities in the fetus – is well known to carry a small risk of miscarriage. But a recent study suggests that risk is even smaller than previously thought.

The test is typically offered to pregnant women older than 35, whose babies have an increased risk for genetic problems such as Down syndrome. It involves the insertion of a hollow needle through the abdomen and into the uterus under ultrasound guidance, so that doctors can withdraw and analyse a sample of fluid from the sac surrounding the fetus. The procedure is usually done during the second trimester, between 15 and 20 weeks of pregnancy.

The risk of having a miscarriage after undergoing amniocentesis in the second trimester is frequently quoted as 0.5 per cent, or one in 200. But when researchers compared pregnancy outcomes in more than 3,000 women who underwent amniocentesis and nearly 32,000 women who did not have the test, they found no significant difference in miscarriage rates.

The rate of miscarriage at less than 24 weeks of pregnancy was one per cent in the amniocentesis group, which was about the same as the 0.94 per cent rate in the comparison group. The difference between these two figures suggests an amniocentesis-related miscarriage rate of 0.06 per cent, or about one in 1,600 – a much lower risk than the traditionally quoted figure.

"Surprisingly lower," Dr. Keith Eddleman, the lead researcher, says of the risk. "All of us who do amnios have had a feeling for a while that the risk is probably lower than what we quote – most people quote 0.5 per cent, or one in 200," says Eddleman, director of maternal-fetal medicine at Mount Sinai School of Medicine in New York. "Now we have the data to show that it is lower. How much lower, exactly, no one knows. In our study it was one in 1,600, but that may be a little too low. ... Probably the realistic thing is that it's lower than one in 200, and what we're telling people now is that it's probably in the ballpark of about one in 1,000."

The current 0.5 per cent risk assessment is based on studies carried out three decades ago, so it's not surprising that the risk may have decreased dramatically. "Everything is different," Eddleman says. "The way we care for patients is different. The technology of ultrasound is different. In the 1970s it wasn't even routine to do amniocentesis under ultrasound guidance. So the whole procedure has changed."