The gush of hormones in a new mother causes a spurt of brain growth.

WASHINGTON — Motherhood may actually cause the brain to grow, not turn it into mush, as some have claimed. Exploratory research published by the American Psychological Association found that the brains of new mothers bulked up in areas linked to motivation and behavior, and that mothers who gushed the most about their babies showed the greatest growth in key parts of the mid-brain.

Contra the press release, I do not think these results address the question of whether women perform at a lower intellectual level while pregnant. All the blood flowing to the fetus or hormones released during pregnancy might cause expectant mom to think fewer complex thoughts. I know women who feel like they got dumber during pregnancy and husbands who agree.

The scientists expected the hormones released after birth to cause brain changes.

Led by neuroscientist Pilyoung Kim, PhD, now with the National Institute of Mental Health, the authors speculated 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 issue of Behavioral Neuroscience.

The motivation to take care of a baby, and the hallmark traits of motherhood, might be less of an instinctive response and more of a result of active brain building, neuroscientists Craig Kinsley, PhD, and Elizabeth Meyer, PhD, wrote in a special commentary in the same journal issue.

Given the existing research literature these speculations were no great leap. For example, prolactin increases nerve myelin sheath production and repair. Prolactin also rises during pregnancy. So prolactin probably alters brain development during pregnancy as well. In rodents neuronal production increases during pregnancy and while raising pups. So these results from humans are not surprising.

Gray matter volume increased in new mothers in several areas of the brain.

The researchers performed baseline and follow-up high-resolution magnetic-resonance imaging on the brains of 19 women who gave birth at Yale-New Haven Hospital, 10 to boys and nine to girls. A comparison of images taken two to four weeks and three to four months after the women gave birth showed that gray matter volume increased by a small but significant amount in various parts of the brain. In adults, gray matter volume doesn’t ordinarily 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).

Women most thrilled by their babies underwent the greatest brain remodeling.

In particular, the mothers who most enthusiastically rated their babies as special, beautiful, ideal, perfect and so on were significantly more likely to develop bigger mid-brains than the less awestruck mothers in key areas linked to maternal motivation, rewards and the regulation of emotions.

The emotional reactions and stimuli women receive from looking at, holding, and smelling their babies probably also contribute to the brain changes these researchers find. There could be a positive feedback loop between the hormone surges, sensory stimuli, emotional reactions, and brain changes.

These changes caused by becoming mothers likely cause permanent changes in brains. Longitudinal studies of women before any have children and then followed for decades could demonstrate whether this is the case. The change should be measurable using pictures and the sounds of babies combined with either MRI scans or to measure emotional responses.

Update: Another study finds oxytocin levels rise in both mom and dad after a baby shows up.

A fascinating new paper by Gordon and colleagues reports the first longitudinal data on oxytocin levels during the initiation of parenting in humans. They evaluated 160 first-time parents (80 couples) twice after the birth of their first child, at 6 weeks and 6 months, by measuring each parents' oxytocin levels and monitoring and coding their parenting behavior.

Three important findings emerged. At both time-points, fathers' oxytocin levels were not different from levels observed in mothers. Thus, although oxytocin release is stimulated by birth and lactation in mothers, it appears that other aspects of parenthood serve to stimulate oxytocin release in fathers.

Higher oxytocin is associated with more parenting behavior.

Finally, the findings revealed that oxytocin levels were associated with parent-specific styles of interaction. Oxytocin was higher in mothers who provided more affectionate parenting, such as more gazing at the infant, expression of positive affect, and affectionate touch. In fathers, oxytocin was increased with more stimulatory contact, encouragement of exploration, and direction of infant attention to objects.

"It is very interesting that elevations in the same hormone were associated with different types of parenting behaviors in mothers and fathers even though the levels of oxytocin within couples were somewhat correlated. These differences may reflect the impact of culture-specific role expectations, but they also may be indicative of distinct circuit effects of oxytocin in the male and female brain," commented Dr. John Krystal, Editor of Biological Psychiatry.

Yet another study found that oxytocin nasal spray increased trust. Oxytocin reduces fear. Parenting probably affects how parents emotionally react the rest of the world. Infant suckling boosts oxytocin release by nerve dendrites. So women who breast feed probably undergo more mental changes than women who bottle feed and therefore it seems reasonable to expect different parenting behavior between breast feeding and bottle feeding parents.