Accuracy of fetal gender determination in the first trimester using three-dimensional ultrasound.

OBJECTIVES: To evaluate the accuracy of three-dimensional (3D) ultrasound in fetal gender assignment in the first trimester. METHODS: A series of pregnant women attending at 11 to 13 + 6 weeks for the nuchal translucency (NT) scan were enrolled into the study. An ultrasound volume of each fetus was obtained and stored for offline analysis. On the reconstructed mid-sagittal plane, the angle between the genital tubercle and an imaginary line passing tangentially through the fetal back (genital angle) was estimated and a receiver-operating characteristics (ROC) curve was constructed to determine the best cut-off for genital angle in fetal male gender determination. Fetal gender was subsequently ascertained in all cases. To calculate the interobserver variability, a second operator repeated the measurements. RESULTS: There were 85 cases included in the study. The genital angle in males was significantly higher than that in females (51.2 +/- 11.3 degrees (n = 36) vs. 18.9 +/- 4.1 (n = 49), P < 0.001). The ROC curve revealed the estimated genital angle to have a high degree of accuracy in fetal gender determination (area under the curve +/- SE = 1.000 +/- 0.001). The best cut-off for male gender determination was found to be between 27 degrees and 29 degrees (sensitivity, 100%; specificity, 98.0%). There was a high degree of correlation between the two operators (r(2) = 0.998; coefficient of variation = 5.4%). CONCLUSIONS: 3D ultrasound is a highly accurate and reproducible tool for fetal gender assignment prior to 14 weeks of gestation.