Influence of maternal body mass index and gestational age on accuracy of first trimester gender assignment

Objective. To determine the influence of maternal body mass index (BMI) and gestational age on the accuracy of image acquisition, first trimester fetal gender determination, and correct assignment. Methods. Women presenting for first trimester aneuploidy risk assessment at 11(0) to 13(6) weeks were prospectively enrolled. A midsagittal view of the fetus including the genital tubercle was obtained. The angle of the genital tubercle was measured with male assigned for angle > 30 degrees, female < 10 degrees, and indeterminate if 10-30 degrees. This was compared with gender at birth. The influence of maternal and pregnancy characteristics on both image acquisition and correct gender assignment were evaluated. Results. A total of 256 women with 260 fetuses undergoing first trimester risk assessment were enrolled. The genital tubercle was identified in 247/260 (95%) of cases. Image acquisition was negatively influenced by increasing maternal BMI and early gestational age (34.8 +/- 7.7 vs. 27.0 +/- 6.1 kg/m(2), p < 0.0001 and 12.3 +/- 0.5 vs. 12.6 +/- 0.5 weeks, p = 0.02). Gender was assigned in 93.1% and correctly matched in 85.8% of fetuses. Positive predictive value (PPV) for male and female fetuses were 88.9% and 79.8%, respectively. Correct gender assignment was more likely in male compared with female fetuses (91.4 vs. 80.5%, p = 0.02). Conclusion. Increasing maternal BMI negatively influences image acquisition during the first trimester for gender determination, but does not decrease the accuracy of correct gender assignment if the image is obtained.