Early midtrimester fetal nuchal thickness: Effectiveness as a marker of Down syndrome

OBJECTIVE: Our purpose was to assess the validity of nuchal thickness in the prediction of Down syndrome in early midtrimester fetuses. STUDY DESIGN: In 1543 consecutive pregnancies undergoing amniocentesis, primarily for advanced maternal age, nuchal thickness was prospectively measured between 13 and 18 weeks and then correlated with the karyotype obtained from amniotic fluid. RESULTS: With the cutoff value suggested in the literature (greater than or equal to 6 mm), 33.3% (6/18) of the cases of Down syndrome would be detected for a 0.1% (2/1424) false-positive rate, with a positive predictive value adjusted to a prevalence of Down syndrome in the general population of 1:3. To increase the sensitivity of the method, the threshold was lowered to 5 mm, achieving a sensitivity of 77.8% (14/18) for a 2.1% (30/1424) false-positive rate and an adjusted positive predictive value of 1:19. CONCLUSIONS: In early midtrimester fetuses decreasing the nuchal thickness threshold to 5 mm substantially enhances the detection of Down syndrome with a reasonable false-positive rate.