First-trimester determination of fetal gender by ultrasound

To assess the accuracy of fetal sex determination at 11-14 weeks of gestation.

Fetal gender assessment by ultrasound was prospectively carried out in 172 singleton pregnancies at 11-14 weeks of gestation immediately before chorionic villus sampling for karyotyping. The genital region was examined in a midsagittal plane and the fetal gender was assigned as male if the angle of the genital tubercle to a horizontal line through the lumbosacral skin surface was greater than 30 degrees and female when the genital tubercle was parallel or convergent (less than 30 degrees) to the horizontal line.

The accuracy of sex determination increased with gestation from 70.3% at 11 weeks, to 98.7% at 12 weeks and 100% at 13 weeks. In the male fetuses, there was a significant increase in the angle of the genital tubercle from the horizontal with crown-rump length. Male fetuses were wrongly assigned as female in 56% of cases at 11 weeks, 3% at 12 weeks and 0% at 13 weeks. In contrast, only 5% of the female fetuses at 11 weeks were incorrectly assigned as male and this false-positive rate was 0% at 12 and 13 weeks.

The clinical value of determination of fetal sex by ultrasound is in deciding whether to carry out prenatal invasive testing in pregnancies at risk of sex-linked genetic abnormalities, because invasive testing would be necessary only in pregnancies with male fetuses. Our results suggest that a final decision on invasive testing for sex-linked conditions should be undertaken only after 12 weeks of gestation.