OBJECTIVE: To assess the behavior of nuchal translucency (NT) in the population of the State of Espírito Santo, Brazil, during weeks 10 to 14 of pregnancy and to establish a reference range for this population. MATERIALS AND METHODS: A transversal study was performed in 853 fetuses which presented normal cariotype or phenotype. All the fetuses were submitted to ultrasound analysis for NT measurement, during routine ultrasound examinations. The Student's "t" test and ANOVA were used in the statistical analysis. The regression test was used to adjust the better mathematical model to translate the NT behavior. RESULTS: Maternal age varied between 14 to 49 years (mean 30.2%), and 22.1% were above 35 years of age. The NT measurement increased with gestational age and the crown-rump length (CRL). In 73 normal fetuses (6.46%) the NT was thicker than 2.5 mm. The better mathematical model was the linear correlation (NT = 0.414 + 0.020 × CRL) which allowed to establish the reference range (percentiles 5, 10, 25, 50, 75, 90 and 95). CONCLUSION: The NT measurement is a manageable method during the usual ultrasonography. NT measurements increase with gestational age during weeks 10 to 14 of pregnancy. The NT measurements seen in the population of the State of Espírito Santo were similar to other population tested.

Keywords: Nuchal translucency, Reference curve, Prenatal diagnosis