Besides recent advances in neonatal care, preterm newborns still develop sex-biased behavioural alterations. Preterms fail to receive placental insulin-like growth factor-1 (IGF-1), a major fetal growth hormone in utero, and low IGF-1 serum levels correlate with preterm poor neurodevelopmental outcomes. Here, we mimicked IGF-1 deficiency of preterm newborns in mice by perinatal administration of an IGF-1 receptor antagonist. This resulted in sex-biased brain microstructural, functional, and behavioural alterations, resembling those of ex-preterm children, which we characterized performing parallel mouse/human behavioural tests. Pharmacological enhancement of GABAergic tonic-inhibition by the FDA-approved drug ganaxolone rescued functional/behavioural alterations in mice. Establishing an unprecedented mouse model of prematurity, our work dissects the mechanisms at the core of abnormal behaviours and identifies a new, readily-translatable therapeutic strategy for preterm brain disorders.