5α-Reduced steroids, including allopregnanolone, suppress neuronal activity and can have neuroprotective effects in the fetus. 5α-Reductases in the placenta may contribute precursors to brain allopregnanolone synthesis. Preterm birth and glucocorticoids, administered for fetal lung maturation or for maternal asthma, may influence reductase expression. The aims of the present study were to evaluate placental 5α-reductase isoform expression during late gestation and to examine fetal sex differences and the effects of glucocorticoid therapies on the expression of these enzymes. Expression of the two 5α-reductase isoenzymes was measured in placental samples, whereas cortisol concentrations were measured in cord blood, from two cohorts. The first cohort consisted of women who delivered preterm and received betamethasone treatment (n = 41); the second cohort consisted of women who delivered at term and were either healthy controls (n = 30) or asthmatics who had used glucocorticoids (n = 24). Placental expression of both isoenzymes increased with advancing gestation and there were marked sex differences in levels of 5α-reductase I (P < 0.05), but not of 5α-reductase II. The expression of both enzymes was positively correlated with cortisol levels (P < 0.05), but there was no effect of recent glucocorticoid exposure. These findings suggest that the preterm neonate may have lower developmental exposure to 5α-reduced steroids and may lack steroid-mediated neuroprotection depending on fetal sex.