Fetal Nasal Bone Predicts Down Syndrome
Ultrasound Finding May Reduce Need for Amniocentesis
Dec. 17, 2002 -- A new method for identifying Down syndrome in fetuses may reduce or even eliminate the need for amniocentesis in many pregnant women. Researchers in the U.K. found that ultrasound images of the nose of the developing fetus during the second trimester can accurately predict the risk of the chromosomal birth defect.
Down syndrome is a genetic condition resulting from an extra chromosome and leads to developmental delays and various congenital heart and gastrointestinal abnormalities.
Lead researcher Kypros Nicolaides, MD, tells WebMD that by combining this and other ultrasound observations with blood testing, the detection rate for Down syndrome is comparable to the invasive genetic test amniocentesis, which carries a risk for miscarriage of about 0.5%.
"About 375,000 amniocenteses are performed every year in the U.S., resulting in about 375 miscarriages," he says. "There are massive medical and emotional costs to this, and we believe that much of this is unnecessary."
For more than a decade, investigators have studied ultrasound as an alternative approach for detecting Down syndrome. Most have focused on the usefulness of measuring fluid behind the neck of the fetus. But a review of studies published last year concluded that while the method is useful, it is not sensitive enough to be a practical test.
In earlier research, Nicolaides and colleagues from London's King's College Hospital found that an underdeveloped fetal nasal bone is a predictor of Down syndrome. The researchers identified the underdeveloped bone in about 70% of fetuses with the chromosomal abnormality, and only about 1% of normal fetuses.
In this study, reported in the journal Ultrasound in Obstetrics and Gynecology, the researchers examined the frequency of finding absent nasal bones in chromosomally normal and abnormal fetuses during the second trimester. They found that the bone was underdeveloped in 62% of the fetuses with Down syndrome, but, again, in only about 1% of normal fetuses.
Nicolaides says the accuracy of the observation is far greater than any other ultrasound method so far studied. The risk of Down syndrome is increased by a factor of 10 in fetuses with thicker-than-normal necks, but it is increased by a factor of 50 in fetuses with the nose anomaly.
"A woman who is 40 has a one in 80 chance of having a baby with Down [syndrome], but if the nose is normal the risk is one-third of that, or one in 240," Nicolaides says. "For a 35-year-old woman with a background risk of one in 300, that risk is lowered to one in 1,000. A woman who knows this would be much less likely to accept the risks of amnio."
Genetic ultrasound expert Anthony Vintzileos, MD, tells WebMD that studies he and several other researchers have conducted on nasal bone development in fetuses suggests that the sensitivity of the observation is lower than that found in the new study. Nasal bone was absent in 41% of the fetuses with Down syndrome that he studied, and other studies also suggest this rate of sensitivity.