Even before a woman gives birth, pregnancy tinkers with the very structure of her brain, several neurologists told me. After centuries of observing behavioral changes in new mothers, scientists are only recently beginning to definitively link the way a woman acts with what’s happening in her prefrontal cortex, midbrain, parietal lobes, and elsewhere. Gray matter becomes more concentrated. Activity increases in regions that control empathy, anxiety, and social interaction. On the most basic level, these changes, prompted by a flood of hormones during pregnancy and in the postpartum period, help attract a new mother to her baby. In other words, those maternal feelings of overwhelming love, fierce protectiveness, and constant worry begin with reactions in the brain.