Congenital adrenal hyperplasia due to P450 oxidoreductase (POR) deficiency (ORD) results in disordered sex development (DSD) in individuals of both sexes. POR provides electrons to CYP17A1 thereby facilitating synthesis of the major androgen precursor dehydroepiandrosterone (DHEA). ORD disrupts this enzymatic step, resulting in deficient synthesis of 5α-dihydrotestosterone (DHT) via DHEA, readily explaining undervirilisation (46,XY DSD) in male ORD neonates. Female virilisation (46,XX DSD) in ORD despite disruption of DHEA synthesis has been proposed to be explained by an alternative pathway to DHT synthesis present in human fetal life only. This pathway also involves CYP17A1 generating androsterone (An) from 17-hydroxyallopregnanolone (5α17HP), the preferred CYP17A1 substrate. The homozygous ORD mutants A287P and H628P are associated with 46,XX DSD and 46,XY DSD, respectively, and we hypothesized that this is explained by distinct effects on alternative androgen pathway activity. In vitro yeast microsomal co-expression of CYP17A1 with both mutants revealed 65% residual conversion of 5α17HP to An in the presence of A287P whereas co-expression of H628P maintained only 16% of wild-type activity. Longitudinal urinary steroid profiling in ORD due to A287P (n=3) in comparison to controls (n=8) documented significantly increased 5α17HP and An excretion during the first three weeks of life. To determine whether there is also evidence for the alternative pathway in normal physiology we performed ex vivo fetal organ culture employing tissues from gestational weeks 7 to 9, the key period of human sexual differentiation, with steroid identification and quantification by tandem mass spectrometry. Results unanimously showed that the human fetal adrenal is capable of all conversions within the proposed alternative pathway, except for the final step, the generation of DHT from androstanediol, which occurred in fetal genital skin. Our results provide conclusive evidence for the existence of an alternative androgen biosynthetic pathway in early human life, with major implications for human physiology and disease.