Screening For All
New guidelines urge every pregnant woman to get early testing for risk of Down syndrome
About a week ago, Melissa Krejdovsky gazed at the tiny profile of her 12-week-old fetus as an ultrasound technician checked for signs of Down syndrome. Ordinarily, the 37-year-old would have undergone amniocentesis, which is recommended for all pregnant women over 35 since they are at increased risk. But her doctor instead suggested the less invasive sonogram and a blood screening test. "I had spotting and was put on bed rest when I was pregnant the first time," says Krejdovsky, who lives with her 4-year-old daughter and husband in Novato, Calif. "I was worried about the small risk of miscarriage from the amnio." The sonogram came back normal, but she is still awaiting the blood test results. She's uncertain what she'll do if they come back indicating Down.
Many women are now getting early screenings for Down, which occurs in as many as 1 in 800 babies and causes mental retardation, facial abnormalities, heart defects, and other problems. Last week, the American College of Obstetricians and Gynecologists recommended that all pregnant women, regardless of age, get tested for the syndrome during the first trimester. Previously, it was offered only during the second trimester, with those over 35 having amniocentesis and younger women getting a quadruple blood test, which measures four Down markers. The blood test, however, misses about 20 percent of Down cases. That's significant since women under 35 give birth to 80 percent of Down syndrome babies.
The early screening is twofold and includes a blood test that detects abnormal levels of two proteins that suggest Down syndrome, and a relatively new ultrasound called nuchal translucency, which looks for excess fluid accumulation at the back of the baby's neck. These flags are about 85 percent reliable at predicting the birth defect. Adding on the quadruple screen increases the accuracy to 95 percent, according to the practice guidelines published in the January issue of Obstetrics & Gynecology.
"The predictive value of prenatal screening has gotten much more accurate," says Nancy Green, medical director of the March of Dimes and an associate professor of pediatrics, obstetrics, and gynecology at Albert Einstein College of Medicine in New York. "In light of that, a practitioner can now offer these options before more invasive diagnostic tests." This should be comforting to women who don't want to risk the possibility of miscarriage from amniocentesis or chorionic villus sampling. With amniocentesis, a needle is inserted into the abdomen to draw out a fetal cell sample to look for chromosomal abnormalities; with CVS, it's more commonly done through the cervix. A November 2006 study by researchers at Mount Sinai School of Medicine found the risk may be as low as 1 in 1,700, but some experts think it could be higher.
Screening can't provide a definitive diagnosis, so those with a positive screen, defined as any risk above 1 in 270, still need to have CVS at 10 to 12 weeks or amniocentesis after the 16th week to know for sure. The advantage of first-trimester testing is that it alerts a woman early in her pregnancy to the risk of Down, allowing plenty of adjustment time before birth or an opportunity for an early abortion. "Results are available sooner in the event that a woman elects to terminate a pregnancy," says Deborah Driscoll, professor and chair of the department of obstetrics and gynecology at the University of Pennsylvania and vice chair of the ACOG committee that issued the new recommendations.