Almost 80% of Down syndrome cases can be detected before birth with a new non-invasive formula using
a ratio of limb measurements, Yale researchers report in the May issue of American Journal of Obstetrics and Gynecology.

This detection method cuts down on the need for amniocentesis, an invasive method of detecting Down syndrome that has a risk of miscarriage. The new formula was devised by the study's first author, Dr. Ray O. Bahado-Singh, associate professor of obstetrics and gynecology at the School of Medicine, and Dr. Utku Oz, research fellow.

Down syndrome is a congenital disorder caused by the presence of an extra 21st chromosome. Those affected have mild to moderate retardation, short stature and a flattened facial profile. Women over age 35 are at a higher risk of giving birth to a child with Down syndrome.

Bahado-Singh's coauthor, Dr. Joshua A. Copel, says an increasing number of women are going to their doctors for non-invasive Down syndrome evaluation.

"Over the last 10 to 15 years, there's been a shift in the mindset of women who come to us for prenatal diagnosis," says Copel, professor of obstetrics and gynecology at Yale. "Far more women are having an ultrasound to get a refined risk estimate. For years, we've been looking for non-invasive ways of determining the risk factors so women can make more informed decisions based on the outcomes."

Two of the common physical characteristics associated with Down syndrome are that the upper arm (humerus) tends to be shorter than average and the skin at the back of the neck (nuchal) tends to be thicker. The differences are extremely subtle, making detection difficult. Ultrasounds done at 16 weeks show a difference of a millimeter.

To try to improve the detection rate, Bahado-Singh, Copel and their research team took the ratio of the upper arm measurement and the neck measurement and multiplied it by the age-related risk to better predict Down syndrome. Using this method, which is called an ultrasonographic biometry algorithm, in their study, they compared 94 cases of Down syndrome and 4,700 normal fetuses. There was a 70.8% detection rate for Down syndrome with about 20% of patients being test-positive.

Copel says it is possible that the results are population-specific to southern Connecticut, so the study needs to be replicated by other researchers. The team's future work will focus on fairly simple measurements that can be done at the bedside to give people better Down syndrome risk estimates. "This will help us target the best candidates for amniocentesis," says Copel. "We can reduce the risk in the vast majority of women, while keeping a high detection rate."

In the near future, Bahado-Singh and the team would like to be able to do an ultrasound, take a few simple measurements and immediately be prepared to give a couple the risk estimates for Down syndrome. "This would mean fewer amniocenteses and a higher percentage of abnormal babies identified," says Copel. These less invasive detection methods could also become cost-effective alternatives to amniocenteses, which cost up to $1,000.

Other researchers on the study included Drs. Chaur-Dong Hsu, Ozgur Deren and Maurice Mahoney.

-- By Karen Peart

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