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 mid-sagittal view of the fetus including the genital tubercle was obtained. The angle of the genital tubercle was measured with male assigned for angle >30°, female <10°, and indeterminate if 10-30°. 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.