The positive predictive value of 1st-trimester nuchal translucency for the diagnosis of fetal aneuploidy is reported to range from 19 to 72% in retrospective series and from 2.8 to 4.8% in prospective studies. In order to analyze the biases accounting for such discrepancies, we retrospectively analyzed a consecutive series of 66 cases of translucency and studied the rate of aneuploidy as a function of sonographic findings and maternal age. The aneuploidy rate was significantly higher in the 23 cases in whom translucency involved the fetal trunk (65%) than in the 43 cases in whom translucency was confined to the nuchal area (16%). This rate was not significantly higher in septated translucencies. These trends persisted after adjustment for maternal age, but the rate of chromosomal anomalies was twice lower in younger mothers. Among the 46 cases in whom the maternal age was < 35 years, 10% of the 30 fetuses with localized nuchal translucency had an abnormal karyotype. In contrast, the rate of aneuploidy was 85% among the 7 cases with diffuse translucency for whom the maternal age was 35 years or above. These findings confirm that both maternal age and size of the translucency should be taken into account to evaluate the risks of aneuploidy.