21-Hydroxylase deficiency is the most common cause of congenital adrenal hyperplasia (CAH), an inherited disorder of steroidogenesis. In its severe form, CAH causes genital ambiguity in females. Molecular genetic analysis of fetal DNA obtained by amniocentesis or chorionic villus sampling is used to diagnose steroid 21-OHD deficiency in utero. Large ongoing studies show that appropriate prenatal treatment of pregnant mothers with dexamethasone is effective and safe for both the fetus and the mother. It reduces ambiguous genitalia in the female affected fetus and thus avoids unnecessary genitoplasty in the newborn female.