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Combination of Tests Can Detect Down Syndrome During First Trimester, Study Says
November 10, 2005
Kaisernetwork.org's Daily Reproductive Health Report
A combination of several tests conducted during the first trimester of pregnancy can better detect fetuses with Down syndrome than standard blood tests done later in pregnancy, according to a NIH-funded study published on Thursday in the New England Journal of Medicine, Bloomberg/New York Times reports (Bloomberg/New York Times, 11/10). Down syndrome is the most common major chromosomal abnormality in the U.S., occurring in about 5,000 infants born annually. Currently, physicians often administer a blood test to pregnant women -- especially women who are older or who have a family history of genetic abnormalities -- in the second trimester around 16 weeks' gestation. If the blood test, known as the quadruple screen, is positive, the woman then undergoes an invasive test called amniocentesis to confirm the diagnosis. The new method consists of performing an ultrasound test, called the nuchal translucency test, and a different blood test in the first trimester between 10 and 13 weeks' gestation (Stein, Washington Post, 11/10). The nuchal translucency test measures the translucent space in the tissue in the back of a developing fetus's neck, which typically is larger in fetuses with Down syndrome because of excess fluid accumulation. The measurement then is put into a formula with the pregnant woman's age and the gestational age of the fetus to determine the likelihood that the fetus has Down syndrome (Kaiser Daily Women's Health Policy Report, 8/5). The corresponding blood test measures the levels of a protein called pregnancy-associated plasma protein A, or PAPP-A, and a hormone called human chorionic gonadotropin, or HCG (Washington Post, 11/10).
Researchers led by Fergal Malone of the Royal College of Surgeons in Dublin, Ireland, administered first-trimester ultrasound and blood tests and the second-trimester quadruple blood test to more than 38,000 pregnant women in the U.S. Researchers then followed the women to see which fetuses had Down syndrome, USA Today reports. The $15 million, eight-year study found that using the combination of ultrasound and blood screening in the first trimester detected 87% of fetuses with Down syndrome, compared with a detection rate of 81% using only the second-trimester blood test. Combining the first- and second-trimester tests, researchers detected 95% of affected fetuses. About 5% of the time, the tests produced false-positive results, meaning a fetus tested positive for Down syndrome but was later determined not to be affected.
The study's findings suggest that fewer pregnant women would need to undergo amniocentesis or another invasive screening test, called chorionic villus sampling, in order to definitively diagnose Down syndrome in their fetuses, Malone said, adding that both tests carry a small risk of infection or miscarriage (Rubin, USA Today, 11/10). In addition, earlier screening would provide women carrying an affected fetus time to decide whether to undergo an abortion or carry the pregnancy to term, according to the Los Angeles Times (Maugh, Los Angeles Times, 11/10). In an accompanying editorial, Joe Leigh Simpson, an OB/GYN professor at Baylor College of Medicine, writes that with the new tests, "[p]regnancy terminations are earlier, more private and far safer than in the second trimester," noting that the maternal death rate for first-trimester abortions is 1.1 per 100,000 abortions, compared with seven to 10 per 100,000 in the second trimester (Bloomberg/New York Times, 11/10). Malone and others doubted that the new tests would result in more abortions, according to the Post (Washington Post, 11/10). Early testing also could cost less, the Wall Street Journal reports. The combination, first-trimester tests cost about $400 or less, compared with about $1,200 for amniocentesis (Tomsho, Wall Street Journal, 11/10). Researchers warned that a limited number of imaging experts have been trained to perform the ultrasound test, which is more complicated than a conventional ultrasound (Los Angeles Times, 11/10). Malone said that many pregnant women do not seek early prenatal care but added that researchers are "trying to get the message out there that people should show up as early as possible for prenatal care," according to the Boston Globe (Goldberg, Boston Globe, 11/10).
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