NORMAL NUCHAL THICKNESS IN THE MIDTRIMESTER INDICATES REDUCED RISK OF DOWN-SYNDROME IN PREGNANCIES WITH ABNORMAL TRIPLE-SCREEN RESULTS

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 greater than or equal to 1:270 risk for Down syndrome on the basis of unconjugated estriol, alpha-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 for 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,XX/47,XXX, 46,XY/47,XY, +7, and 46,XX,11q-. The sensitivity of an abnormal nuchal thickness (greater than or equal to 6 mm) for detecting Down syndrome was four in eight (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 greater than or equal to 1:100 on the basis of the receiver-operator characteristic plot. Of 390 cases with a normal nuchal thickness and a serum risk estimate < 1:100, there were no cases of Down syndrome (0/390 vs 8/253, 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.