Finding Out Faster
Jan. 8, 2001 -- Pregnant at 34 with her third child, my sister Jill wanted to know whether her baby would have Down syndrome. It's the most common chromosomal birth defect and can cause mental retardation and heart problems. But Jill feared amniocentesis, the diagnostic prenatal test that could give her an answer.
So she decided to try a new, experimental test: a first trimester, prenatal screening that combines ultrasound and a blood test to determine the risk of Down syndrome as well as trisomy 18, a less common but most often fatal defect.
The First Trimester Maternal Serum Biochemistry and Fetal Nuchal Translucency Screening is currently available, sometimes free-of-charge, at 13 medical centers across the United States as part of a federally funded clinical trial. Researchers predict it will prove more accurate than the standard prenatal test -- the triple screen, a blood test -- and be more widely available in two years.
Best taken 10 to 13 weeks into a pregnancy, the new screening test involves testing a small amount of a pregnant woman's blood to measure several chemicals that may be above or below normal if there's a birth defect. Ultrasound is used to measure a fluid-filled area behind the fetus' neck. Extra fluid in this area has been linked to birth defects. These results plus the mother's age are used to calculate a woman's risk. Previous studies have found the test detects up to 90% of the cases of Down syndrome or trisomy 18.
Compared to amniocentesis, the new screen is done earlier, it's less invasive, and it doesn't increase the risk of miscarriage. An amniocentesis is done during the second trimester, involves a needle inserted into the abdomen, and slightly increases a woman's normal risk of miscarriage -- by 0.5%.
But amniocentesis and the other major diagnostic test -- chorionic villus sampling, or CVS -- are more accurate.
Remember: A screening test defines the risk of having a problem, while a diagnostic test determines if there is a problem. Amniocentesis is 99% accurate for Down syndrome and more than 90% accurate for many other less common serious chromosome abnormalities, as well as for physical birth defects such as spina bifida.
"No matter how sensitive the [new] test is, there will be some women who will be assured by the test and still have a baby with Down syndrome," says Ronald Wapner, MD, director of maternal/fetal medicine at Thomas Jefferson University Hospital in Philadelphia, which is leading the clinical trial. Results [from the trial] are due in 2001.
Compared to the triple screen, the new test is done earlier and may be more accurate. The triple screen is not done until 16 to 18 weeks of pregnancy and detects only 60% to 80% of fetuses with Down syndrome.
"The biggest advantage is the earlier time they'll have the information," Wapner says. By 12 weeks of pregnancy, a woman will know if she needs a diagnostic test, "which will allay the anxiety for most women a lot earlier."
During her recent second pregnancy, Hildrun Passas, 37, of Havertown, Pa., learned about the new test from friends in her native Germany, where it is more widely available. Passas received a rare inconclusive result from the new test and went on to have amniocentesis, which showed no problems. She still believes the new test could be an important medical advance. "I really feel very strongly that doctors should know more about it and tell women about it," says Passas, who has a doctorate in biology.
The new test is among several emerging alternatives to amniocentesis, which still is offered routinely to women age 35 or over because they have a greater risk of having a child with Down syndrome. Doctors generally consider amniocentesis safe and low-risk. But women don't take it until the second trimester, when they look and feel very pregnant. If they get bad news, their decision about whether to end the pregnancy is all the more difficult and public.
Researchers hope the new test will help more women avoid amniocentesis. If it proves more accurate, the new test also would detect more defects, especially among pregnant women under age 35. Because these women have the most babies, they have the most babies with Down syndrome.
Again, the new test isn't perfect. It has a 5% screen-positive rate. This means 5% will receive a positive result suggesting higher risk and be offered amniocentesis or CVS. The vast majority will go on to have normal babies.
For my sister, this was problematic. Her first trimester screen showed low risk. Her second trimester screen showed higher risk. She had the amniocentesis after all. It showed no problems.
Researchers are looking into these "inconsistencies," Wapner says.
Here, then, is a wrap-up of the options:
Maternal Serum Screening Tests: During the second trimester (15 to 18 weeks), a small sample of the woman's blood is tested. The alpha-fetoprotein test (AFP) measures one substance and is 80% to 90% accurate for detecting open neural tube defects such as spina bifida, 25% for Down syndrome. The triple screen measures AFP plus two other substances and is 80% to 90% accurate for open neural tube defects, 60% to 80% for Down syndrome. The screen-positive rate is 5%.
First Trimester Maternal Serum Biochemistry and Fetal Nuchal Translucency Screening: During the first trimester or soon after (9 to 14 weeks), a small blood sample is tested to measure several chemicals and an ultrasound measures a fluid-filled area behind the neck. These results combined with maternal age determine risk. One study showed accuracy of up to 90% for detecting Down syndrome and trisomy 18. Considered experimental, the screen is undergoing clinical trials in the United States. The screen-positive rate is 5%.
Ultrasound: During the second trimester (typically 18 weeks), sound waves are used to create pictures of the fetus. Both the two-dimensional and three-dimensional ultrasounds are used to search for structural abnormalities. The 3-D, which adds a computer-generated third dimension, may confirm suspected problems. Part screening, part diagnostic, the accuracy rate is variable.
Amniocentesis: During the second trimester (15 to 18 weeks), a small sample of amniotic fluid is removed using a needle inserted through the abdomen. This diagnostic test is 99% accurate for Down syndrome, more than 90% accurate for other less common defects including spina bifida. It slightly increases the normal risk of miscarriage, by 0.5%.
Chorionic Villus Sampling (CVS): During the first trimester (10 to 12 weeks), cells are removed from the placenta by inserting a narrow plastic tube into the vagina or a needle into the abdomen. This diagnostic test is slightly less accurate than amniocentesis and the miscarriage risk is slightly greater -- 0.5% to 1% above normal risk. Rare cases of limb deformity have been reported, especially when done before 10 weeks. CVS does not test for open neural tube defects.
Betsy Rubiner is based in Des Moines, Iowa, and specializes in writing about children and families. Her work has appeared in The New York Times, The Philadelphia Inquirer, and The Boston Globe, among other publications.
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