Fetal gender assignment by first-trimester ultrasound

Objective: Ultrasound 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.

Methods: Fetal 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 degrees , and female gender if the genital tubercle was parallel or convergent (<10 degrees ) 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.

Results: Gender 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.

Conclusion: Prenatal 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.