Early after giving birth, a woman's brain goes through big chemical shifts—including more oxytocin—to get her ready for caring for a baby, but those same shifts can also make her more prone to feeling down or anxious. Some scientists think mild mood dips might have evolved to push new moms to seek extra help when conditions are tough. The paper reviews animal and human data on these brain changes and how they could lead to postpartum mental health issues.

One of the most perplexing issues in clinical psychology is why the early postpartum period, supposed to be one of the happiest moments of a woman's life, is often associated with a significantly higher incidence of mental health problems. This manuscript advocates the idea that pervasive mood disturbances during the early postpartum period can be understood by exploring their proximate neurobiological mechanisms and ultimate adaptive functions. At the biological level, alterations in mood states are thought to proximately result from pregnancy- and motherhood-induced neurobiological changes, especially in the monoamine and glucocorticoid systems in various limbic areas (e.g., the hippocampus and hypothalamus). On the one hand, these changes must occur in order to prepare a new mother to better face the challenge of motherhood both physiologically and behaviorally, as they are critically important for increasing prolactin and oxytocin production to prepare for parturition and milk production, enhancing attentional and emotional processing of infant-related cues, and allowing normal onset and expression of maternal behavior. On the other hand, these changes also increase the mothers' stress response and vulnerability to experience mood disturbances. Evolutionarily, mild mood disturbances have been proposed to have the evolved functions of conserving resources and soliciting more social support to help raise a child when the unfavorable circumstances demand. The ultimate force influencing postpartum mood disturbances is the evolutionary pressure that promotes inclusive fitness of the mother. Clinical postpartum mental disorders most often develop in individuals with increased genetic and environmental risks for mental disorders. According to this proximate-ultimate theory, postpartum mood disturbances could be considered as either an unfortunate "by-product" or a potentially adaptive behavioral strategy of normative neurobiological changes from pregnancy to the early postpartum period. They clearly serve an evolutionary purpose to ameliorate unfavorable social and environmental conditions. This article reviews relevant animal and human studies on functional brain changes in postpartum females and articulates how these changes could contribute to the development of postpartum mental disorders.