PRO & CON.

Should Down syndrome screening standards be changed so that amniocentesis and chorionic villus sampling are offered on the basis of maternal serum screening results, not maternal age?

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 fewer women 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[6]:979-85, 2000). During that period, there was nearly a threefold increase in the number of women aged 3549 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.