The authors of the new study, which was published in Nature Neuroscience, suspect the reductions they’ve detected may be a side-effect of “synaptic pruning,” which also happens to humans at age three and again during adolescence.
In teenagers, this clean-up is thought to fine-tune neural connections and networks, supporting the “specialization of brain circuitry, which is critical for healthy cognitive, emotional and social development,” the researchers explain in their paper.
In pregnant women, they speculate, the gray matter shrinkage may function as a similar kind of decluttering, but one that would optimize a woman’s ability to sense what’s going on for other people, what their moods are, and when the emotional weather is shifting. This change would benefit a woman on the cusp of becoming a mother, as it would allow her to better anticipate and understand her baby’s needs.
The authors say that shrinking plays out different in rodents. A mother’s brain changes in ways that tweak her ability to forage and hunt for food. Evolutionary pressure may have put more emphasis on social processes in humans.
To conduct their research, the co-lead authors, Elseline Hoekzema, a senior researcher in the Brain & Development Lab at Leiden University in the Netherlands, and Erika Barba-Müller, a psychologist at Autonomous University of Barcelona, first put 25 women who were planning to become mothers, and 19 of their male partners through high-resolution brain scans. The same men and women were scanned again when the pregnancies were over. The control group contained 20 women who had never given birth, and 17 male partners.
Notably, the scientists turned up no evidence of cognitive impairment in the women who became pregnant. The women did trend slightly lower on verbal word memory tasks after giving birth, but not by an amount the scientists considered statistically significant.”Sometimes less is more,” Hoekzema told. “Loss of volume does not necessarily translate to loss of function.”
The volume reductions they found were primarily located in the prefrontal and temporal cortex, regions of the brain thought to be associated with social cognition and other functions. Two years later, the scientists conducted follow-up scans with the mothers who had not had a second pregnancy (11 women) and found their brain volume levels had not recovered, except in the hippocampus, the brain area associated with memory. It would take a much longer study to determine whether the changes in the brain are permanent.
Notably, women whose MRI images showed the most brain re-sculpting also appeared to have developed a stronger bond with their children, as measured by two assessment tools: postpartum scans that captured images of the women’s brains as they looked at photos of their babies, and standardized questionnaires on mother-infant attachment.
That said, experts who were not part of the study told CNN that there were other possible explanations for the results. For instance, David Van Essen, an investigator with the National Institute of Health, said that a surge of myelin, a protective membrane that coats nerve fibers and boosts the speed of neural communication, could read as a change in gray matter volume in fMRI scans. Kim Yonkers, a psychiatrist and professor of obstetrics and gynecology at Yale School of Medicine, pointed out that the areas of the brain where loss occurred were also associated with functions unrelated to social cognition, like depression and pain management. A little brain shrinking could help woman forget the pain of giving birth, she proposed.
Previous studies have suggested that pregnant women are better able to understand emotions encoded in faces, especially during late-stage pregnancy, while other scientists believe pregnancy turns women into better organizers. The potential upsides to “baby brain” keep adding up.
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