Nuchal translucency measurement at different crown-rump lengths along the 10- to 14-week period for Down syndrome screening.
OBJECTIVE
To evaluate the screening accuracy for Down syndrome of nuchal translucency (NT) measurement at different crown-rump length (CRL) subgroups along the 10- to 14-week period.


METHODS
NT was classified 'enlarged' if greater than or equal to 1.5 and 2.0 multiples of the regressed median. Accuracies for Down syndrome (formula=[(TP + TN)/(TP + TN + FP + FN)]x100, where TP: true positive, TN: true negative, FP: false positive, FN: false negative) were evaluated in four classes of CRL: 38-44 mm, 45-54 mm, 55-70 mm, and 71-84 mm, and compared.


RESULTS
Of 20 743 fetuses, 20 611 were with no chromosomal abnormalities and 132 were with Down syndrome. Down syndrome fetuses with enlarged NT were 99 (greater than or equal to 1.5 MoM) and 86 (greater than or equal to 2.0 MoM). Sensitivity decreased with gestation, while specificity increased, resulting in increasing likelihood ratios with gestation for each of the CRL groups (8.1, 14.1, 16.3, 17.1 with the use of the 1.5 MoM cut-off, and 13.2, 27.1, 50.1, 84.1 for the 2.0 MoM cut-off). The accuracy increased with gestation (89%, 95%, 95%, 96% with the use of the 1.5 MoM cut-off, and 94%, 97%, 98%, 99% for the 2.0 MoM cut-off, for each of the CRL groups), differences being statistically significant between periods in half of the comparisons.


CONCLUSIONS
Although sensitivity of NT assessment for Down syndrome screening decreased as gestation advanced from the 10th to the 14th week, accuracy showed a remarkable increase. These changes should be taken into account in defining and improving the Down syndrome screening policies.
Zoppi
Maria A
MA
Department of Obstetrics and Gynecology, Prenatal and Preimplantation Genetic Diagnosis, Fetal Therapy, Ospedale Microcitemico, Cagliari, Italy.
Ibba
Rosa M
RM
Floris
Marcella
M
Manca
Fabiola
F
Axiana
Carolina
C
Monni
Giovanni
G
D000293:Adolescent; D000328:Adult; D018568:Crown-Rump Length; D004314:Down Syndrome / Q000000981:diagnostic imaging* / Q000473:pathology; D005260:Female; D006801:Humans; D008403:Mass Screening; D008875:Middle Aged; D048208:Nuchal Translucency Measurement / Q000592:standards*; D011237:Predictive Value of Tests; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D012680:Sensitivity and Specificity