EARLY MIDTRIMESTER FETAL NUCHAL THICKNESS - EFFECTIVENESS AS A MARKEROF DOWN-SYNDROME

OBJECTIVE: Our purpose was to assess the validity of nuchal thickness in the prediction of Down syndrome in early midtrimester fetuses. STUD Y DESIGN: In 1543 consecutive pregnancies undergoing amniocentesis, pr imarily for advanced maternal age, nuchal thickness was prospectively measured between 13 and 18 weeks and then correlated with the karyotyp e obtained from amniotic fluid. RESULTS: With the cutoff value suggest ed 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 prevale nce of Down syndrome in the general population of 1:3. To increase the sensitivity of the method, the threshold was lowered to 5 mm, achievi ng 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: I n early midtrimester fetuses decreasing the nuchal thickness threshold to 5 mm substantially enhances the detection of Down syndrome with a reasonable false-positive rate.