Mother's Feelings Toward Babies Tied to Bigger Midbrains

Motherhood may in fact cause the brain to grow, not turn it into mush, as some have maintained. Exploratory research found that the brains of new mothers bulked up in regions associated with motivation and behavior, and that mothers who enthused the most about their babies showed the greatest growth in key areas of the midbrain.

Led by neuroscientist Pilyoung Kim, Ph.D., now with the U.S. National Institute of Mental Health (NIMH; Bethesda, MD, USA), the investigators hypothesized that hormonal changes right after birth, including increases in estrogen, oxytocin, and prolactin, may help make mothers' brains susceptible to reshaping in response to the baby. Their findings were published in the October 2010 issue of the journal Behavioral Neuroscience.

The motivation to take care of a baby, and the characteristic traits of motherhood, might be less of an innate reaction and more of a result of active brain building, neuroscientists Craig Kinsley, Ph.D., and Elizabeth Meyer, Ph.D., wrote in a special commentary in the same journal issue.

The researchers performed baseline and follow-up high-resolution magnetic-resonance imaging (MRI) on the brains of 19 women who gave birth at Yale-New Haven Hospital (New Haven, CT, USA) to 10 boys and 9 girls. A comparison of images taken two to four weeks and three to four months after the women gave birth revealed that gray matter volume increased by a small but significant amount in various areas of the brain. In adults, gray matter volume does not typically change over a few months without significant learning, brain injury or illness, or major environmental change.

The areas affected support maternal motivation (hypothalamus), reward, and emotion processing (substantia nigra and amygdala), sensory integration (parietal lobe), and reasoning and judgment (prefrontal cortex). Specifically, the mothers who most passionately rated their babies as special, beautiful, ideal, perfect, and so on were considerably more likely to develop bigger mid-brains than the less enthralled mothers in key areas linked to maternal motivation, rewards and the regulation of emotions.

The mothers averaged just over 33 years in age and 18 years of school. All were breastfeeding, nearly half had other children, and none had serious postpartum depression.

Although these early findings require replication with a larger and more representative sample, they raise intriguing questions about the interaction between mother and child (or parent and child, since fathers are also the focus of study). The intense sensory-tactile stimulation of an infant may trigger the adult brain to grow in key areas, allowing mothers, in this case, to "orchestrate a new and increased repertoire of complex interactive behaviors with infants,” the authors wrote. Expansion in the brain's "motivation” area in particular could lead to more nurturing, which would help babies survive and thrive physically, emotionally, and cognitively.

Additional research using adoptive mothers could help "tease out effects of postpartum hormones versus mother-infant interactions,” said Dr. Kim, and help resolve the question of whether the brain changes behavior or behavior changes the brain--or both.

The authors noted that postpartum depression might involve reductions in the same brain areas that grew in mothers who were not depressed. "The abnormal changes may be associated with difficulties in learning the rewarding value of infant stimuli and in regulating emotions during the postpartum period,” they said. Further study is expected to clarify what happens in the brains of mothers at risk, which may lead to improved interventions.

In their "Theoretical Comment,” Drs. Kinsley and Meyer, of the University of Richmond (VA, USA), connected this research on human mothers to similar basic research findings in laboratory animals. All the scientists agreed that additional studies might show whether increased brain volumes are due to growth in nerve cells themselves, to longer and more complex connections (dendrites and dendritic spines) between them, or to bushier branching in nerve-cell networks.

Related Links:
National Institute of Mental Health
University of Richmond