Anna Harp and her husband, Doug Harp (left) of Binghamton, N.Y., look at a sonogram of their baby with Ronald Wapner, a Jefferson perinatologist, and Patti Morgan, a sonographer. (April Saul / Inquirer Staff Photogrpaher)

When she got pregnant at 39, Anna Harp knew her age raised the risk that her baby would have Down syndrome, the most common chromosomal birth defect.

But the college professor from Binghamton, N.Y., also had concerns about trying to detect the disorder before birth.

The standard blood test used to screen all pregnancies misses 40 percent of fetuses with Down syndrome, which causes mental retardation, heart defects, and distinctive facial features. Yet the definitive diagnostic test - an amniocentesis, which she would automatically be offered because of her age - occasionally triggers a miscarriage. Worse, the diagnosis cannot be made until the second trimester. At that point, an abortion, if Anna and her husband, Doug, were to consider it, would be more medically and emotionally difficult.

Having gone through two ectopic pregnancies and then infertility treatment, the Harps began searching for a better way.

They found it at Thomas Jefferson University Hospital, where researchers are leading a federally funded, national study that promises to provide more reliable screening for Down syndrome as early as 10 weeks after conception. The screening method will also improve detection of trisomy 18, a less common chromosomal birth defect that is most often fatal.

The new method, which is already being widely used in Britain, appears to detect about 90 percent of fetuses with trisomy 18 or Down syndrome - about 40 percent more than with standard screening.

On Thursday, just 12 weeks into her pregnancy, Anna Harp underwent the experimental screening, which involves a blood test and a special ultrasound exam.

The ultrasound image dramatically magnified the Harps' tiny, 21/2-inch fetus, revealing a crucial feature - a normal neck. Only in the last five years have scientists discovered that the backs of the necks of Down syndrome fetuses are unusually thick.

By itself, the neck thickness suggested there was a good chance - better than 60 percent - that the fetus did not have Down syndrome or trisomy 18. This week, the Harps will receive a computer analysis that combines the ultrasound results and her age with two blood protein measurements. One of the proteins was recently discovered to be an early biochemical indicator of Down syndrome.

"I feel reassured," said Doug Harp, a computer programmer. "I just want the blood-test results so we can be absolutely sure."

"We've gone through so much to get to this point," added his wife. "We didn't want to take the risk of an amniocentesis if we could avoid it."

Jefferson perinatologist Ronald Wapner, who is leading the study with Jefferson geneticist Laird Jackson, said the Harps were typical.

"For the vast majority of women, this gives them early reassurance," he said.

With that reassurance, many women who now undergo amniocentesis could be spared the procedure while even more affected fetuses could be identified. And the new method can be used between 10 and 14 weeks of pregnancy, compared with 14 to 18 weeks with the current standard screening blood test.

Wapner explained that improved detection is particularly important for pregnant women under 35. Because there are so many of them, they give birth to most Down syndrome babies. Yet because their age does not put them at high risk, they are not automatically offered a diagnostic test.

Only about 5 percent of all women screened with the new method have results that indicate they need a definitive diagnostic test. All such women receive genetic counseling. Those who are in their first trimester, when amniocentesis is too dangerous, may choose to undergo chorionic villus sampling, which involves analyzing cells taken from the placenta. (It causes a miscarriage about 0.5 percent of the time, the same as amniocentesis later in pregnancy.) Beyond 12 weeks, they may opt for amniocentesis.

The study began about a year ago and aims to screen 60,000 women at 13 medical centers across the country. Final results should be available in two years, Wapner said.

Although most women are referred by their obstetricians, some, like Anna Harp, find their way to Jefferson in Center City after reading about the study on its Web site.

"We've had calls from as far away as Saudi Arabia," said Maureen McGee, a Jefferson research nurse. "They're so glad to have earlier information through non-invasive testing."

She added: "Many women say it doesn't matter if I'm high risk because I won't do anything [to end the pregnancy], but I just want to know."

© 1998 Philadelphia Newspapers Inc.