Optimal nuchal skin-fold thresholds based on gestational age for prenatal detection of Down syndrome.
OBJECTIVE
We attempted to determine optimal nuchal skin-fold thresholds throughout the second trimester of pregnancy for the detection of Down syndrome.


METHODS
Thresholds of 5, 6, and 7 mm were prospectively tested in 8138 consecutive midtrimester pregnancies. The pregnancies were divided into early (14 to 18 weeks) and late (19 to 24 weeks) second-trimester time frames. The pregnancies were also subdivided by maternal age, and the same thresholds were tested for efficacy in the women < 35 years old versus those > or = 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 thickness of > or = 5 mm was the optimal threshold. Measurements of this magnitude were found in 2.9% of pregnancies and produced a sensitivity of 42% (11/26) for the detection of Down syndrome. When adjusted for the incidence of Down syndrome in the general population, the 5 mm threshold had a positive predictive value of 1 in 48. In the 19 to 24 week gestational time frame, > or = 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 predictive value was 1 in 38. The sensitivity of nuchal skin-fold thickness for Down syndrome detection was similar in women < 35 and > or = 35 years old. Positive predictive value was better in the women > or = 35 years old because of the higher prevalence of affected fetuses in this group.


CONCLUSIONS
Whereas definitive diagnostic testing should still be offered to all women with risk factors for a fetus with trisomy 21, differential nuchal skin-fold thresholds based on gestational age are effective for the detection of Down syndrome in pregnancies at low risk.
Department of Obstetrics and Gynecology, Washington University School of Medicine, St. Louis, MO.
D L
DL
Gray
J P
JP
Crane
D000328:Adult; D004314:Down Syndrome / Q000000981:diagnostic imaging*; D005260:Female; D005865:Gestational Age; D006801:Humans; D008423:Maternal Age; D009333:Neck / Q000000981:diagnostic imaging*; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D018566:Pregnancy, High-Risk; D011296:Prenatal Diagnosis; D012885:Skinfold Thickness; D014463:Ultrasonography