OBJECTIVE: Our purpose was to determine whether nuchal thickness measurement can identify the euploid fetuses in midtrimester pregnancies at increased risk for Down syndrome on the basis of maternal age and serum screening.

STUDY DESIGN: Nuchal thickness was obtained prospectively in 651 consecutive fetuses at 14 to 21 weeks' gestation and at ≥1:270 risk for Down syndrome on the basis of unconjugated estriol, ⍺-fetoprotein, and human chorionic gonadotropin levels. The risk of Down syndrome with a normal nuchal thickness was determined. A receiver-operator characteristic curve was used to determine a serum-based risk threshold below which the risk of Down syndrome was low. The prevalence of Down syndrome in fetuses with both a normal nuchal thickness and a below-serum-risk threshold was compared with prevalence in either those above threshold risk or with an abnormal nuchal thickness.

RESULTS: There were eight cases of trisomy 21 and one case each of 46,XX47,XXX, 46,XY47,XY, + 7, and 46,XX,11q−. The sensitivity of an abnormal nuchal thickness (≥6 mm) for detecting Down syndrome was four ineight (50%) (95% confidence interval 15.3% to 84.6%). The risk of Down syndrome was significantly increased with an abnormal compared with a normal nuchal thickness, four in 13 (30.8%) versus four in 638 (0.6%), p < 0.0001. A risk threshold was defined at ≥1:100 on the basis of the receiver-operactor characteristic plot. Of 390 cases with a normal nuchal thickness and a serum risk estimate <1:100, there were no cases of Down syndrome (0390 vs 8253, p = 0.002).

CONCLUSION: Normal nuchal thickness significantly reduces the risk of Down syndrome and may help reduce the number of amniocenteses done for abnormal triple screen results.