PRO & CON.
Dr. James F.X. Egan is an associate professor of obstetrics and gynecology and pediatrics at the University of Connecticut, Farmington.
YES The current practice of offering routine amniocentesis to those aged 35 or older without first preforming a maternal serum screen or triple screen is based on demographics from the 1970s when there were fewer women who were delaying childbirth.
In a recently published study, my colleagues and I at the University of Connecticut analyzed natality statistics in the United States from 1974 to 1997 (Obstet. Gynecol. 96:979-85, 2000). During that period, there was nearly a threefold increase in the number of women aged 35-49 who bore children. Meanwhile, the number of 16-week-old fetuses estimated to have Down syndrome increased by less than twofold. Many more women would be offered amniocentesis today, compared with 30 years ago. This discrepancy suggests that age alone is not the best way of screening pregnancies for Down syndrome.
Using a mathematical model and historical data, we found that the maternal serum screen had greater sensitivity and lower false-positive rates in detecting Down syndrome, compared with basing risk on age and conducting amniocenteses accordingly.
In 1997, performing the maternal serum screen on all women would have helped avoid nearly 155,000 amniocenteses, resulting in almost 800 fewer procedure-related losses. At the same time, 1,556 more Down syndrome fetuses would have been identified by the triple test than by maternal age.
Patients would need to be warned about the limits of maternal serum screening. In women aged 35 and older, the serum test would miss 11%-12% of Down syndrome cases. Patients will decide if this risk is worth the benefit of reducing the number of amniocenteses.
Some of those cases of missed Down syndrome may be picked up by ultrasound. My colleagues and I plan to present data at the upcoming annual meeting of the Society of Maternal-Fetal Medicine on the capability of ultrasound to do just that. Improved serum testing with the addition of inhibit-A to the triple test should also increase the detection rate.
Maternal serum screening is also limited in its ability to detect chromosomal abnormalities other than Down syndrome. The test frequently, but not always, detects trisomy 18, Turner's syndrome, triploidy, and trisomy 16. It does not detect trisomy 13, although this is generally quite obvious on ultrasound.
Our findings concur with those of two earlier studies. Their collective conclusions make a persuasive argument that it's time for practitioners and policy makers to address whether routine Down syndrome screening should continue to be based on maternal age.
Dr. Mark Evans is professor and chair of obstetrics and gynecology and professor of human genetics at the Medical College of Pennsylvania, Philadelphia.
NO I do not agree that we should make it standard practice to conduct invasive testing such as amniocentesis or chorionic villus sampling (CVS) on the basis of maternal serum screening results rather than on maternal age.
The problem with existing routine noninvasive screening technology, such as maternal serum screening, is that it presently can be used only in the second trimester.
While I have no problem with women being offered the choice of maternal serum screening, requiring that women have abnormal maternal serum screening before amniocentesis is not a policy change that I can support. The reason is that amniocentesis and CVS can diagnose Down syndrome or other genetic abnormalities by week 10 or 11. If pregnant women have a second-trimester screening test first, before amniocentesis, the whole process is pushed back 1-2 months and they don't reap the advantage of having a clear and private first-term answer.
When a woman is 10 weeks pregnant, she can make a decision about whether to have an abortion based on genetic test results, without anyone else ever having to know about it. But by the time the results from the maternal serum screening test and then a subsequent amniocentesis are available--around 19 weeks' gestation--the woman is already visibly pregnant.
While amniocentesis and CVS admittedly involve a small risk of pregnancy loss, their findings are definitive. By contrast, even by the most optimistic accounts, maternal serum screening results have only a 70%-80% detection rate. So at least 20% of the cases will be missed.
Should a policy favoring maternal serum screening over maternal age as the determinant for conducting amniocentesis become standard, it would be very easy to imagine payers requiring an abnormal screening test before they would pay for a patient to have an invasive test. That would in effect eliminate access to coverage for first-trimester testing and be a major step backward for women.
California's Medi-Cal program has already implemented such a restrictive policy. Under it, women aged 35 and older are offered the choice between having second-trimester amniocentesis or second-trimester maternal serum screening.
However, if a woman chooses the maternal serum screen and it isn't positive, the program won't cover an amniocentesis. So she would then have to pay the nearly $1,000 in expenses out-of-pocket.
On the other hand, noninvasive screening tests before amniocentesis or CVS screening would be a reasonable option when such tests are eventually available for routine first-trimester use.
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|Publication:||Family Practice News|
|Article Type:||Brief Article|
|Date:||Feb 1, 2001|
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