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

OBJECTIVE: Our purpose was to determine whether nuchal thickness measu rement can identify the euploid fetuses in midtrimester pregnancies at increased risk for Down syndrome on the basis of maternal age and ser um screening. STUDY DESIGN: Nuchal thickness was obtained prospectivel y in 651 consecutive fetuses at 14 to 21 weeks' gestation and at great er than or equal to 1:270 risk for Down syndrome on the basis of uncon jugated estriol, alpha-fetoprotein, and human chorionic gonadotropin l evels. The risk of Down syndrome with a normal nuchal thickness was de termined. A receiver-operator characteristic curve was used to determi ne a serum-based risk threshold below which the risk for Down syndrome was low. The prevalence of Down syndrome in fetuses with both a norma l nuchal thickness and a below-serum-risk threshold was compared with prevalence in either those above threshold risk or with an abnormal nu chal 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 sensit ivity of an abnormal nuchal thickness (greater than or equal to 6 mm) for detecting Down syndrome was four in eight (50%) (95% confidence in terval 15.3% to 84.6%). The risk of Down syndrome was significantly in creased 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 rec eiver-operator characteristic plot. Of 390 cases with a normal nuchal thickness and a serum risk estimate < 1:100, there were no cases of Do wn syndrome (0/390 vs 8/253, p = 0.002). CONCLUSION: Normal nuchal thi ckness significantly reduces the risk of Down syndrome and may help re duce the number of amniocenteses done for abnormal triple screen resul ts.