Study: First-Trimester Test for Down Syndrome Is Most Accurate

Nov. 9, 2005 — -- "Do you want a boy or a girl?"

This question, commonly asked of pregnant women, is often met with the same response -- "I just want a healthy child."

But, unfortunately, birth defects occur in about 4 percent of all babies born each year in the United States, according to the March of Dimes. One of the most common genetic causes is Down syndrome, a chromosomal birth defect that causes a flat facial profile, low muscle tone and varying levels of mental retardation.

Since the 1970s, several prenatal screening tests could detect Down syndrome, but none of them offered 100 percent accuracy. Newer methods still don't carry perfect accuracy, but a new study of more than 38,000 pregnant women shows that a particular method -- two blood tests and an ultrasound done during the first trimester -- is more accurate than any other method.

The study, conducted by researchers with the First- and Second-Trimester Evaluation of Risk group, appears Thursday in the New England Journal of Medicine.

Screening tests, which are based on complicated projections based on the blood tests and ultrasound, do not tell a women definitively if her baby has Down syndrome -- it only gives the probability.

For example, Heidi and Albert Miller, a couple in Robinson, Pa., had early prenatal testing when Heidi became pregnant last year. They learned that their child had a one in 870 chance that he would be born with Down syndrome.

"This was very reassuring," Heidi said, who was 37 at the time. "The chances were so low that we considered this really good news."

An accurate way to screen for Down syndrome in the earlier stages of pregnancy is important, said Dr. John Larsen, chairman of Obstetrics and Gynecology at George Washington University in Washington, D.C.

"The time … two to three months [earlier] … is huge for the person who wants privacy," Larsen said in an e-mail. "Obviously pregnant women become subject to much public attention."

The Millers' ability to find out their baby's risk of Down syndrome is not a common one -- of the 3 million births in the U.S. each year, only 15 percent of their mothers undergo first trimester screening, according to Dr. Mark Evans, president of the Fetal Medicine Foundation of America. That's because a specially trained person must conduct the ultrasound test.

"Right now, the most sophisticated [doctors' offices] and patients already know this," Evans said. "But for the vast majority of practicing obstetricians, this is not the standard -- though it should be."

Because she was in her late 30s, Heidi was considered to be at an "advanced maternal age," which meant she had a slightly higher chance of having a baby with Down syndrome. But obstetricians point out there is a common misconception that only older women give birth to children with the disorder.

"The majority of infants with Down syndrome, approximately 70 percent, are born to women under the age of 35," said Dr. Richard Fischer, a professor in the Obstetrics and Gynecology Department at Robert Wood Johnson Medical School in New Jersey.

Earlier testing does raise some fears that it will increase abortions, said Ann Scheidler, executive director of the Pro-Life Action League. However, she said that screening does have some benefits.

"If a woman is going to use the knowledge to help better prepare herself for how she's going to take care of her baby, the test could be helpful," Scheidler said.

Heidi Miller felt similar to this. Although their baby was born healthy, she said they "wanted to know so we could prepare ourselves."

"We knew we'd keep our baby regardless," she said.

Dr. Tracy Wimbush is an emergency physician at Massachusetts General Hospital in Boston.