| Screen All Pregnancies for Down Syndrome, Doctors Say -
11-01-2007, 03:34 AM
For decades, pregnant women 35 and older have routinely been tested for chromosomal abnormalities that might affect their fetus, most notably Down syndrome. Younger women were tested much less often because the risk of birth defects was low and invasive procedures like amniocentesis carried risks of their own.
But now the American College of Obstetricians and Gynecologists is recommending Down syndrome screening for all pregnant women, regardless of their age.
The change was prompted in part by new, less invasive screening techniques, including an ultrasound exam that helps assess risk as early as 11 weeks into a pregnancy — though not as accurately as the invasive procedures, amniocentesis and chorionic villus sampling.
But the new recommendations also address a demographic fact of life: while older women are at greater individual risk of conceiving a child with Down syndrome, most babies with the disorder are born to young women, simply because they give birth in far greater numbers.
“It’s been pretty much ingrained in obstetricians’ minds that 35 is the cutoff age,” said Dr. Deborah A. Driscoll, chairwoman of the obstetrics department at the University of Pennsylvania and a lead author of the new recommendations. “We’re trying to move away from that.
“The take-home message,” Dr. Driscoll continued, “is that all women, regardless of age, should be offered Down syndrome screening, and any woman should then have the option of diagnostic testing.”
Down syndrome is the most common chromosomal abnormality, affecting 1 in 800 live births. Children with the syndrome, who are born with an extra chromosome, display unusual facial features, suffer from moderate to severe mental retardation, and are at high risk for congenital heart defects and other health problems.
The risk of conceiving a child with a chromosomal abnormality does increase with age. For Down syndrome, the risk is 1 in 1,250 at age 25 but 1 in 106 at age 40.
The new guidelines erase any age distinction when it comes to offering screening and testing: older women may choose to undergo only a screening test, which predicts the degree of risk but may be inaccurate, while younger women may opt for a more invasive, more expensive but also more precise diagnostic test, like amniocentesis.
In explaining the new recommendation, several physicians said the cutoff age of 35 had always been somewhat arbitrary. “Many women are unhappy with it, because it doesn’t mean anything to them; it’s kind of being put upon them,” said Dr. James Goldberg, a former chairman of the obstetrics college’s committee on genetics, who helped develop the new guidelines.
For example, Dr. Goldberg said, a 29-year-old woman and her partner might now choose amniocentesis instead of a blood test. In the past, the more invasive procedure was seldom recommended for younger women because it could sometimes result in miscarriage. Now the risk is considered to be quite low, and in any event, Dr. Goldberg said, for some couples “losing a normal pregnancy secondary to the procedure is not as problematic as the birth of a Down syndrome child, so they’re willing to take that risk.”
The new guidelines, published in the January issue of Obstetrics and Gynecology, outline a dizzying array of screening options that individual physicians will need to choose from. Not all tests will be available to all patients, and one of the newer ultrasound exams requires special training and may not be available at all centers.
The ultrasound test, called a nuchal translucency test, involves measuring the fluid that accumulates in the back of the neck of a fetus, just under the skin. There is a strong association between the thickening of the neck in this area and the risk of Down syndrome, and studies have found that this measurement, combined with two blood tests done in the first trimester, can detect 82 percent to 87 percent of Down syndrome cases.
“Pediatricians have known for a long time that children with Down syndrome have excessive skin folds in the back of the neck,” said Dr. Ray Bahado-Singh, a board member of the Society for Maternal-Fetal Medicine. “This will also pick up the most common sex chromosome abnormality, Turner syndrome, which classically presents with the thickening of the neck.”
The catch is that the ultrasound must be conducted early in the pregnancy, at 11 to 13 weeks of gestation, when some women have yet to realize they are pregnant or to see an obstetrician.
“It’s a very narrow window,” said Dr. Driscoll of the obstetrics college, emphasizing the importance of getting very early prenatal care. “If a patient isn’t seen for obstetric care for the first 13 weeks of pregnancy, she would miss out on an opportunity to have this test.”
A number of blood tests can screen for chromosomal abnormalities in the second trimester, including the “triple screen” and the “quadruple screen,” which look for abnormal levels of biochemical markers, proteins and hormones. But while the quadruple screen detects up to 81 percent of Down syndrome cases, a so-called integrated approach, which uses both the nuchal translucency and the quad screen as well as another blood test, has a detection rate of 94 percent to 96 percent. The screens may be less sensitive for other chromosomal abnormalities.
Officials at the March of Dimes welcomed the new guidelines, saying they would help provide women with better information earlier in the progression of a pregnancy.
“The new guidelines are much better for the broader group of women who are having babies,” said Dr. Nancy Green, the organization’s medical director, “and that’s public health: doing the most good for the biggest number, the good in this case being the information people need to make decisions.” |
Linear Mode
