Several features on ultrasound scans that doctors thought were warning signs of Down syndrome are almost always harmless, leading many women with normal fetuses to undergo unnecessary amniocentesis, researchers say.
The analysis of 56 previous studies casts doubt on the reliability of ultrasound in detecting Down syndrome in the womb.
Pregnant women routinely undergo blood tests and ultrasound scans to check for abnormalities in the fetus. When warning signs are spotted on the ultrasound image, women are often advised to undergo amniocentesis, in which a needle is used to draw fluid from the womb for analysis. Amniocentesis itself carries a small risk of causing a miscarriage.
In this latest research, doctors looked at seven markers sometimes spotted on ultrasound scans that are thought to increase the risk of Down's, a form of retardation caused by a certain abnormality.
Only one of those markers - a shaded area suggesting a thickening at the back of the neck - was found to be reliable enough indicator to justify an amnio, said Dr. Rebecca Smith-Bindman, a radiologist at the University of California at San Francisco who led the study.
“Even then, only 3 percent of fetuses will be affected by Down syndrome, but this risk is sufficiently high that a woman might consider invasive testing with amniocentesis,” Smith-Bindman said.
“The six other markers were certain brain cysts; shortening thigh and upper-arm bones; bright spots on the bowel or heart; a high fluid levels in the kidneys. About 10 percent to 14 percent of pregnant women have one of these markers, but fewer than 1 percent have Down's babies,” Smith-Bindman said.
“For women whose blood tests are normal, if your baby is found to have any of these isolated markers, your baby is almost certainly normal and you shouldn't be concerned about it and undergo an amnio because of it,” she said. In 1998, the latest year for which figures are available, 112,778 amnios and 2.5 million ultrasounds were performed in the United States. Smith-Bindman would not estimate how many amnios are unnecessary, but it's “clearly a lot.”
“Ultrasounds that show an obvious structural abnormality, such as heart defects that are common in Down syndrome, should not be discounted,” she said.
One in 800 to 1,000 U.S. babies is born with Down syndrome. Women over 35 and those who have already had an affected baby face an increased risk.
Routine prenatal blood tests done in the second trimester are considered a more accurate diagnostic tool for Down's. Amniocentesis is the most definitive test, but it can trigger a miscarriage in about one in 200 to one in 400 cases.
In fact, the risks of miscarriage from amniocentesis are greater than the risks that a woman with normal blood tests but one of the ultrasound markers will have a Down syndrome baby, according to the analysis.
Smith-Bindman's analysis is based on studies published between 1980 and 1999 involving 132,295 fetuses, including 1,930 with Down syndrome. It appears in Wednesday's Journal of the American Medical Association.
Dr. Joe Leigh Simpson, a Down syndrome expert and chairman of obstetrics and gynecology at Baylor College of Medicine, said the findings are not surprising and underscore the weaknesses using ultrasound to detect Down syndrome.
Simpson said the older studies analyzed may have used outdated ultrasound machines, but he added that even when the most accurate scanners are operated by the most experienced technicians, second-trimester ultrasound is still not the best way to diagnose Down syndrome.
This article first appeared in issue #10 of Down Syndrome Amongst Us