OBJECTIVE The hypothesis was fetal sex determination by ultrasound at 11-14 weeks' gestation has sufficient accuracy to be clinically relevant. METHODS Fetal sex assessment by transabdominal ultrasound was performed in 567 fetuses at 11-14 weeks' gestation (CRL: 45-84 mm). A mid-sagittal view of the genital region was obtained. The angle of the genital tubercle to a horizontal line through the lumbosacral skin surface was measured. The fetus was assigned male sex if the angle was > 30°, and female sex if the genital tubercle was parallel or convergent (< 10°). At an intermediate angle of 10-30°, the sex was not assigned. The results were divided into three categories based on gestational age: 11 + 2 to 12 + 1, 12 + 2 to 13 + 1, and 13 + 2 to 14 + 1 weeks' gestation. To establish its accuracy, the first trimester fetal sex determination was compared to fetal sex determined on a mid-second trimester ultrasound. RESULTS Sex assignment was successful in 534/683 (78%) of the cases. The overall accuracy of fetal sex assignment across all gestational ages studied was 94.4%. It was 88.3%, 94.7%, and 98.6% at 11 + 2 to 12 + 1, 12 + 2 to 13 + 1, and 13 + 2 to 14 + 1 weeks' gestation, respectively. CONCLUSIONS Prenatal sex assignment at the time of first trimester ultrasound screening has a high accuracy rate. The accuracy improved with increasing gestational age, which suggests that if clinically important decisions, such as chorionic villus sampling, are to be made based on fetal sex, they should be delayed until the latter part of the first trimester.