“This is what we call an aspect of almost the obsessive-compulsive behaviors during the very first few months after the baby’s arrival,” maternal brain researcher Pilyoung Kim told me. “Mothers actually report very high levels of patterns of thinking about things that they cannot control. They’re constantly thinking about baby. Is baby healthy? Sick? Full?”

“In new moms, there are changes in many of the brain areas,” Kim continued. “Growth in brain regions involved in emotion regulation, empathy-related regions, but also what we call maternal motivation—and I think this region could be largely related to obsessive-compulsive behaviors. In animals and humans during the postpartum period, there’s an enormous desire to take care of their own child.”

There are several interconnected brain regions that help drive mothering behaviors and mood.

Of particular interest to researchers is the almond-shaped set of neurons known as the amygdala, which helps process memory and drives emotional reactions like fear, anxiety, and aggression. In a normal brain, activity in the amygdala grows in the weeks and months after giving birth. This growth, researchers believe, is correlated with how a new mother behaves—an enhanced amygdala makes her hypersensitive to her baby’s needs—while a cocktail of hormones, which find more receptors in a larger amygdala, help create a positive feedback loop to motivate mothering behaviors. Just by staring at her baby, the reward centers of a mother’s brain will light up, scientists have found in several studies. This maternal brain circuitry influences the syrupy way a mother speaks to her baby, how attentive she is, even the affection she feels for her baby. It’s not surprising, then, that damage to the amygdala is associated with higher levels of depression in mothers.

Amygdala damage in babies could affect the mother-child bond as well. In a 2004 Journal of Neuroscience study, infant monkeys who had amygdala lesions were less likely to vocalize their distress, or pick their own mothers over other adults. A newborn’s ability to distinguish between his mother and anybody else is linked to the amygdala.

Activity in the amygdala is also associated with a mother’s strong feelings about her own baby versus babies in general. In a 2011 study of amygdala response in new mothers, women reported feeling more positive about photos depicting their own smiling babies compared with photos of unfamiliar smiling babies, and their brain activity reflected that discrepancy. Scientists recorded bolder brain response—in the amygdala, thalamus, and elsewhere—among mothers as they looked at photos of their own babies.