If a fetus has a greater-than-normal amount of swelling at the back of the neck (nuchal translucency), there is a high likelihood that the baby will have Down Syndrome. The current invasive second trimester screening for Down syndrome via amniocentesis identifies only 50 to 60 percent of Down syndrome fetuses. First trimester ultrasound screening for nuchal translucency (NT), either alone or with serum markers of the condition, can identify more Down syndrome fetuses with fewer procedure-related losses and is more cost effective than the currently used second trimester amniocentesis. That's the conclusion of a study supported in part by the Agency for Healthcare Research and Quality (National Research Service Award training grant T32 HS00086).
However, if the combined first trimester ultrasound and blood screening strategy were recommended today, there would neither be enough laboratory capacity for the blood screening nor enough ultrasonologists who are trained to perform NT, caution the researchers who conducted the study. They calculated that the benefit of NT ultrasound alone to identify each additional Down syndrome case would outweigh the cost by nearly five to one. The benefit of adding to the ultrasound a first-trimester serum screen (for pregnancy-associated plasma protein A and free beta-human chorionic gonadotropin fragments) would still outweigh the cost by nearly two to one for each additional Down syndrome fetus identified.
These calculations were based on a screening decision model the researchers developed to apply to the entire population of the United States and the 4 million infants born here each year. They designed a decision tree to compare four possible screens for Down syndrome: current second-trimester expanded AFP test during amniocentesis (low AFP levels correlate with Down syndrome); first trimester NT screen; first trimester serum screen; and combined first trimester NT and serum screen. The combined screen identified 3,833 Down syndrome fetuses, the NT screen alone 3,413, and the first-trimester serum screen, 2,993 compared with 2,446 identified by the currently used expanded AFP screen.
More details are in "Nuchal translucency and first trimester biochemical markers for Down syndrome screening: A cost-effectiveness analysis," by Aaron B. Caughey, M.D., M.P.P., M.P.H., Miriam Kuppermann, Ph.D., M.P.H., Mary E. Norton, M.D., and A. Eugene Washington, M.D., M.Sc., in the November 2002 American Journal of Obstetrics and Gynecology 187,
pp. 1239-1245.
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