AbstractObjectiveUltrasound determination of fetal sex can benefit decision‐making regarding invasive prenatal testing in pregnancies at risk of sex‐linked genetic abnormalities. The aim of this study was to assess the accuracy of fetal sex determination by ultrasound at 12–14 weeks of gestation in a large cohort.MethodsFetal gender assessment by transabdominal ultrasound was performed in 656 singleton pregnancies at 12–14 weeks of gestation. The genital region was examined in the mid‐sagittal plane. The angle of the genital tubercle to a horizontal line through the lumbosacral skin surface was measured. The fetus was assigned male gender if the angle was > 30°, and female gender if the genital tubercle was parallel or convergent (<10°) to the horizontal line. At an intermediate angle of 10–30 degrees the gender was not determined. Crown–rump length (CRL) was measured in all cases.ResultsGender assignment was possible in 613 of the 656 (93%) fetuses. Gender identification according to CRL was feasible in 85%, 96% and 97% of the fetuses at gestational ages of 12 to 12 + 3, 12 + 4 to 12 + 6 and 13 to 13 + 6 weeks, respectively. Phenotypic sex was confirmed in 555 newborns. The accuracy of male gender assignment in this group was 99–100% at all ages, and that of female gender assignment was 91.5% at 12 to 12 + 3 weeks, 99% at 12 + 4 to 12 + 6 weeks and 100% at 13 to 13 + 6 weeks.ConclusionPrenatal gender assignment by ultrasound has a high accuracy rate at 12–14 weeks. These results indicate that invasive testing can probably be carried out in fetuses identified as males at this gestational age. However, in fetuses identified as female at a CRL of <62.6 mm, despite the relatively high 91.5% accuracy rate, the decision regarding invasive testing should be postponed until a higher CRL is achieved. Copyright © 2006 ISUOG. Published by John Wiley & Sons, Ltd.