To the Editor: Dr Smith-Bindman and colleagues1 concluded that ultrasound screening in the second trimester is not an efficacious screening method for fetal Down syndrome.
Their analysis and conclusions focused primarily on 6 isolated ultrasonographic markers for Down syndrome, and they did not fully consider associated fetal structural malformations, which occur in 28% of fetuses with Down syndrome or multiple markers.2 Table 3 of the article shows that when multiple markers and structural abnormalities are considered together, ultrasound screening has a sensitivity of 69% and a false-positive rate of 8%. The PPVs cited also are underestimated because their prevalence rates for Down syndrome are low. In 1997, there was an overall second-trimester prevalence of Down syndrome in the United States of 1:504. For women aged 35 years or older, the prevalence was 1:134.3 Using these prevalence rates, ultrasound screening had a PPV of 1.68%, that is, 1 in 60 screen-positive women had an affected pregnancy.
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