Cortical volume reductions in men transitioning to first-time fatherhood reflect both parenting engagement and mental health risk.

Perinatal reductions in gray matter volume have been observed in human mothers transitioning to parenthood, with preliminary evidence for similar changes in fathers. These reductions have been theorized to support adaptation to parenting, but greater investigation is needed. We scanned 38 first-time fathers during their partner's pregnancy and again after 6 months postpartum, and collected self-report data prenatally and 3, 6, and 12 months postpartum. Significant gray matter volume reductions were observed across the entire cortex but not the subcortex. Fathers who reported stronger prenatal bonding with the unborn infant, and planned to take more time off from work after birth, subsequently showed larger cortical volume decreases. Larger reductions in gray matter volume also emerged among fathers who reported stronger postpartum bonding with the infant, lower parenting stress, and more time spent with their infant. Larger volume reductions predicted more postpartum sleep problems and higher levels of postpartum depression, anxiety, and psychological distress, controlling for prenatal sleep and mental health. Volume reductions were smaller among fathers whose infants were older at the postpartum scan, indicating potential rebound. These results suggest that perinatal gray matter volume reductions might reflect not only greater parenting engagement but also increased mental health risk in new fathers.