A new first-trimester procedure can determine Down syndrome risk with a simple blood test and specialized ultrasound.
One of the top questions on Julie Hoegee’s mind when she found out she was pregnant at age 34 was whether she should have an amniocentesis. “There was just something scary about a big needle going into my belly,” says the Los Angeles mother of 8-month-old Charlie. “And I knew there was a slight miscarriage risk.” So when Hoegee’s nurse-midwife and doctor told her about combined first-trimester screening, a blood test and new ultrasound technique that can reliably determine risk for chromosomal abnormalities, such as Down syndrome, as early as week 11 of pregnancy, she jumped on it. “I knew it wasn’t 100 percent definitive, but it was close enough for me,” Hoegee says.
Don’t feel out of the loop if you’ve never heard of first-trimester screening; only about 10 percent of pregnant women currently receive the test, says Mary E. D’Alton, M.D., chairwoman of the OB-GYN department at Columbia University College of Physicians and Surgeons in New York and the lead researcher in a major study of the screening that was sponsored by the National Institutes of Health. But she says that number is expected to jump to 25 percent in the next couple of years. “Our objective is to offer the test to all pregnant women,” D’Alton says.
Old vs. new: comparing the tests
Today, many pregnant women receive a prenatal blood test called the triple marker, or expanded alpha-fetoprotein (AFP), between weeks 16 and 18 to look for chromosomal abnormalities, as well as neural-tube defects such as spina bifida. This test detects about 65 percent of fetuses with Down syndrome. However, in the late 1990s, a fourth marker (“quad” screening) that increases the detection rate to 80 percent was added. Which test you get depends on your physician’s, HMO’s or state agency’s preference, says Joe Leigh Simpson, M.D., chairman of the OB-GYN department at Baylor College of Medicine in Houston and an expert in first-trimester screening.
The newest procedure is performed at 11 to 12 weeks and involves a test of the mother’s blood and a specialized ultrasound that measures nuchal translucency (NT), the amount of fluid in skin folds at the back of the fetus’s neck. These results, combined with the mother’s age, help determine a fetus’s risk of Down syndrome. In a study of 33,000 pregnancies, D’Alton found the detection rate to be about the same as the second-trimester quad screening: 83 percent vs. 85 percent. However, the false-positive rate for the first-trimester screening was just 6 percent, compared with 9 percent for the later one. A blood test and ultrasound are still recommended in the second trimester to look for neural-tube defects and other malformations.
What the results mean and what comes next
If your first-trimester screening is positive, you may want to follow up with one of two older procedures—chorionic villus sampling (CVS) or amniocentesis. Both can determine with nearly 100 percent accuracy whether the fetus has Down syndrome. With CVS, which can be performed in the first trimester, a needle is inserted into the uterus to obtain a sample of the placenta. Amniocentesis, which involves using a needle to extract amniotic fluid for analysis, cannot be performed until at least week 15.
Done properly, both procedures pose only a small risk of miscarriage or other problems. “When performed by an experienced doctor, the procedure-related risk of amnio and CVS is about one in 400,” Simpson says. (Ideally, your physician should have done several hundred or more of these procedures.)
Researchers are divided on whether women should receive both first- and second-trimester Down syndrome tests. In research D’Alton conducted, doing so yielded the highest detection rate ever for a noninvasive test—94 percent—but the false-positive rate was 11 percent.
However, waiting for confirmation in the second trimester eliminates the first trimester test’s main advantage. “If a woman decides to terminate her pregnancy, doing so in the first trimester offers her greater privacy and access to a provider,” Simpson says. “Waiting until later increases risk of complications—and even maternal death—tenfold.”
For more information on prenatal testing, go to www.fitpregnancy.com/yourpregnancy/89