Objective: To define the best fixed cutoff point for nuchal translucency, with the assistence of the ROC curve, and its accuracy in screening all fetal aneuploidy and trissomy 21 in a South American population.

Type of study: Validation of diagnostic test.

Setting: The study was carried out at the State University of Campinas, Campinas, Brazil.

Participants: 230 patients examined by ultrasound at two tertiary-level private centers, at 10 to 14 weeks of gestation.

Diagnostic test: The participants consisted of all those patients who had undergone ultrasound imaging at 10-14 weeks of gestation to measure nuchal translucency and who had had the neonatal karyotype identified.

Results: Prevalence of chromosomal defects - 10%; mean age - 35,8 years; mean gestational age - 12 weeks and 2 days; nuchal translucency (NT) thickness - 2,18 mm. The best balance between sensitivity and specificity were values that were equal to or higher than 2,5mm for overall chromosomal abnormalities as well as for the isolated trissomy 21. The sensitivity for overall chromosomal abnormalities and trissomy 21 were 69,5% and 75%, respectively, and the positive likelihood ratios were 5,5 and 5,0, respectively.

Conclusion: The measurements of nuchal translucency was found to be fairly accurate as an ultrasound marker for fetal abnormalities and measurements equal to or higher than 2,5 mm were the best fixed cutoff points.

Keywords: Aneuploidy, Ultrasound, Chromossomes, Fetus, Prenatal Diagnosis.