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.
Gray
D L
DL
Department of Obstetrics and Gynecology, Washington University School of Medicine, St. Louis, MO.
Crane
J P
JP
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