Smaller hippocampal volume is associated with psychiatric disorders. Variations in hippocampal volume are discussed as both a consequence of the neurotoxic effects of stress and as a pre-existing condition leading to increased vulnerability for cognitive and emotional impairments. To investigate whether early experience can account for variability in hippocampal volume in adulthood (vulnerability hypothesis), we assessed the relationship between birth weight and hippocampal volume in 44 subjects. The reported quality of maternal care in early childhood, as evaluated by the Parental Bonding Inventory, was used as index of the quality of the postnatal environment. Hippocampal volume was assessed from magnetic resonance images using a manual segmentation protocol. We show that birth weight significantly predicts hippocampal volume in adulthood only in female subjects reporting low maternal care. The results suggest that the postnatal environment modulates the neurodevelopmental consequences of prenatal risk and that this effect is sex-specific.