WEDNESDAY, Nov. 8, 2005 (HealthDay News) -- Researchers say that by using a test called first-trimester combined screening, they can now detect Down syndrome earlier in pregnancy than ever before.
The combined screen includes an ultrasound test called a nuchal translucency test, plus tests that look for two proteins in maternal blood.
"This test is a big step forward for pregnant women because, on its own, it picks up 87 percent of cases of Down syndrome, and does so extremely early in pregnancy -- 11 or 12 weeks -- which is sufficiently early that women can find out important pregnancy information before being visibly pregnant," said Dr. Fergal Malone, chairman of the department of Obstetrics and Gynecology at the Royal College of Surgeons in Dublin, Ireland.
Malone was lead author of the study, which compared the new test to standard second-trimester screening, called quadruple screening. His team published their findings in the Nov. 10 issue of the New England Journal of Medicine.
According to the March of Dimes, the average 25-year-old American woman has about a one-in-1,250 chance of giving birth to a baby with Down syndrome. That risk climbs with age, so that by the time a woman reaches 40, her odds of giving birth to a Down baby has risen to about one in 100.
Second-trimester quadruple screening is currently the standard Down syndrome screening test used in the United States, according to the study. This test, which focuses on maternal blood proteins, is generally performed 15 weeks to pregnancy. If test results are abnormal, many women choose to undergo amniocentesis, which can diagnose Down syndrome. This means that a woman is often 18 or 19 weeks pregnant when she gets her amniocentesis results, Malone said.
He said the nuchal translucency test was first developed in the early 1990s and can only be done at between 11 and 13 weeks' gestation. Using ultrasound, a technician measures the thickness of the back of the fetus' neck. A difference of just a fraction of a millimeter, along with the presence of two proteins -- pregnancy-associated plasma protein A and the free beta subunit of human chorionic gonadatropin -- can signal the presence of Down syndrome.
If this combined screening test is positive, many women follow-up with chorionic villus sampling (CVS), which like amniocentesis can diagnose Down syndrome, but does so at an earlier stage of pregnancy. The optimal time for this test is 11 weeks. Before then, the fetus is too small, and after 13 weeks, the fetus is too large for the ultrasonographer to get an accurate picture.
To compare combined screening to standard quadruple screening, Malone and colleagues from centers in the United States, Ireland and the U.K. performed both tests on more than 38,000 pregnant women. One hundred and seventeen women in this group had babies with Down syndrome.
Both tests had an overall 5 percent false-positive rate, meaning that they picked up "cases" of abnormality that were in fact normal. The first-trimester combined screening was more effective in detecting Down syndrome, the researchers report.
Rates of detection for combined screening were 87 percent, 85 percent and 82 percent for tests done at 11, 12 and 13 weeks, respectively. Traditional quadruple screening had a detection rate of 81 percent.
The researchers stressed that the nuchal translucency test is very difficult to perform, and requires special training.
"Typical measurements in most women are only about 1 millimeter, and fractions of a millimeter differences in this measurement can make a huge difference to the risk of Down syndrome quoted to the woman," said Malone, who added that even the best-trained sonographers can't obtain measurements in 7 percent to 8 percent of cases.
Still, Malone said he is offering combined screening to his patients.
"The major benefit is that women can now get a great degree of reassurance about the health of their baby, and the likelihood that they are going to have a normal pregnancy, much earlier than previously. Additionally, if bad news is discovered, and a patient decides to undergo a pregnancy termination, such terminations are much safer for the woman at 12 weeks compared with 20 weeks," said Malone.
"This study confirms that first-trimester combined screening is a very precise way to screen for Down," said Dr. Allen Hogge, chairman of the department of obstetrics and gynecology and a geneticist at Magee-Women's Hospital at the University of Pittsburgh Medical Center.
"The problem at this point is that it's not going to be widely available for a while," he said. "It's also just a screening test. It's not a yes or no answer."
Malone added that while the combined screening test isn't widely available, it is being performed in many major medical centers in the United States and Europe. However, he pointed out that many insurers, including Medicaid, aren't currently reimbursing for this test yet.
"Patients may have to pay for this screening test themselves, or they and their doctors may have to get involved with complex claims procedures. Hopefully, the data provided in [our] trial will convince more insurers that not only is this a valid test, but it is probably the single most efficient screening test for Down syndrome that is available today," said Malone.
The American Pregnancy Association has more information on prenatal testing.