A new mother’s body goes through many changes—among them, key parts of her brain get bigger, according to research reported in October’s Behavioral Neuroscience. And the more these areas grow, the greater the mother-infant bond seems to be.

Structural changes in animal brains, says National Institutes of Health neuroscientist Pilyoung Kim, are critical to getting mothers to take care of their offspring. Similar changes in human mothers, she observes, might be necessary for attentive parenting and ultimately forming long-term emotional bonds, and now there is evidence suggesting that possibility. Using MRI, Kim and her colleagues at Yale University and the University of Michigan at Ann Arbor produced detailed maps of the brains of 19 new mothers a few weeks after they gave birth. At around the same time, the researchers asked mothers to select words from a list of positive descriptors such as “beautiful,” “perfect” and “special” to describe how they felt about their babies and about their experience of parenting.

When the scientists mapped the mothers’ brains again about three months later, some areas had grown, including the hypothalamus, amygdala and substantia nigra—regions that animal studies suggest are involved with caring for, learning about and forming positive feelings toward newborns. The planning and decision-making part of the brain, the prefrontal cortex, also grew. In addition, mothers who initially chose more of the positive words to describe their feelings about their babies showed more brain growth. The investigators do not yet know what causes what—if brain growth leads to more positive feelings, or vice versa—but the results indicate for the first time a connection between mothers’ subjective feelings and physical changes in the brain. Kim says they are planning more studies to investigate the phenomenon, including one that will look for similar changes in fathers.