Discrepancy in fetal sex assignment between cell free fetal DNA and ultrasound

Noninvasive prenatal testing utilizing free fetal DNA is commonly used in pregnancy to screen for trisomy 13, 18, 21 and also fetal sex aneuploidy. We report on two cases of discrepancy between phenotypic and genotypic sex and potential medical implications. In our first case, a patient with known male gender via cell free fetal DNA (cffDNA) testing had an ultrasound at 18 weeks' gestation, which identified Dandy-Walker malformation and female-appearing ambiguous genitalia. As Dandy-Walker malformation could not be considered isolated in the presence of ambiguous genitalia, this finding allowed for more complete counseling of the parents as well as extensive genetic workup. Our second case involved a fetus with intrauterine growth restriction diagnosed by ultrasound and normal-appearing female genitalia. After birth, adrenal insufficiency was diagnosed and chromosome analysis identified normal male chromosomes. These two cases showed that fetal sex determination by cffDNA can be used as a tool for earlier identification of affected pregnancies, allowing for parental decision-making, genetic testing and earlier intervention.