Progesterone and its neuroactive metabolite, allopregnanolone, are present in high concentrations during pregnancy, but drop significantly following birth. Allopregnanolone influences foetal arousal and enhances cognitive and behavioural recovery following traumatic brain injury. Inhibition of allopregnanolone synthesis increases cell death in foetal animal brains with experimental hypoxia. We hypothesised that complications during pregnancy, such as early or preterm loss of placental steroids and intrauterine growth restriction (IUGR), would disrupt the foetal neurosteroid system, contributing to poor neurodevelopmental outcomes. This study aimed to investigate the effects of chronic inhibition of allopregnanolone synthesis before term and IUGR on developmental processes in the foetal brain. Guinea pig foetuses were experimentally growth restricted at mid-gestation and treated with finasteride, an inhibitor of allopregnanolone synthesis. Finasteride treatment reduced foetal brain allopregnanolone concentrations by up to 75% and was associated with a reduction in myelin basic protein (MBP) (P = 0.001) and an increase in glial fibrillary acidic protein expression in the subcortical white matter brain region (P < 0.001). IUGR resulted in decreased MBP expression (P < 0.01) and was associated with a reduction in the expression of steroidogenic enzyme 5α-reductase (5αR) type 2 in the foetal brain (P = 0.061). Brain levels of 5αR1 were higher in male foetuses (P = 0.008). Both IUGR and reduced foetal brain concentrations of allopregnanolone were associated with altered expression of myelination and glial cell markers within the developing foetal brain. The potential role of neurosteroids in protecting and regulating neurodevelopmental processes in the foetal brain may provide new directions for treatment of neurodevelopmental disorders in infants who are exposed to perinatal insults and pathologies.