OPTIMAL NUCHAL SKIN-FOLD THRESHOLDS BASED ON GESTATIONAL-AGE FOR PRENATAL DETECTION OF DOWN-SYNDROME

OBJECTIVE: We attempted to determine optimal nuchal skin-fold threshol ds throughout the second trimester of pregnancy for the detection of D own syndrome. STUDY DESIGN: Thresholds of 5, 6, and 7 mm were prospect ively tested in 8138 consecutive midtrimester pregnancies. The pregnan cies were divided into early (14 to 18 weeks) and late (19 to 24 weeks ) second-trimester time frames. The pregnancies were also subdivided b y maternal age, and the same thresholds were tested for efficacy in th e women < 35 years old versus those greater than or equal to 35 years old. RESULTS: There were a total of 32 fetuses with trisomy 21 in the study population. From 14 to 18 weeks' gestation, a nuchal fold thickn ess of greater than or equal to 5 mm was the optimal threshold. Measur ements of this magnitude were found in 2.9% of pregnancies and produce d a sensitivity of 42% (11/26) for the detection of Down syndrome. Whe n adjusted for the incidence of Down syndrome in the general populatio n, the 5 mm threshold had a positive predictive value of 1 in 48. In t he 19 to 24 week gestational time frame, greater than or equal to 6 mm appeared to be the optimal threshold, yielding a positive screen rate of 3.7% with a sensitivity of 83% (5/6). The adjusted positive predic tive value was 1 in 38. The sensitivity of nuchal skin-fold thickness for Down syndrome detection was similar in women < 35 and greater than or equal to 35 years old. Positive predictive value was better in the women greater than or equal to 35 years old because of the higher pre valence of affected fetuses in this group. CONCLUSIONS: Whereas defini tive diagnostic testing should still be offered to all women with risk factors for a fetus with trisomy 21, differential nuchal skin-fold th resholds based on gestational age are effective for the detection of D own syndrome in pregnancies at low risk.