When a fetus is identified as 46,XX based on cell-free DNA but male or atypical genitalia are identified on a second trimester anatomy ultrasound, amniocentesis for karyotype and CYP21A2 mutation testing should be performed before assuming that the fetus has CAH, as a variety of conditions can explain these findings including testing errors [63]. The gene discussed is CYP21A2; the disease is congenital adrenal hyperplasia.