What might the loss mean?  There are three possibilities, said Paul Thompson, a neuroscientist at the University of Southern California who was not involved in the study. “The most intuitive is that losing gray matter is not beneficial, that later on there may be negative consequences.”  Or, he said, it could be just a “neutral” reflection of pregnancy-related “stress, diet, lack of sleep.”  A third possibility is that the loss is “part of the brain’s program for dealing with the future,” he said. Hormone surges in pregnancy might cause “pruning or cellular adaptation that is helpful,” he said, streamlining certain brain areas to be more efficient at mothering skills “from nurturing to extra vigilance to teaching.”  The study strongly leans toward the third possibility.  “We certainly don’t want to put a message out there on the lines of ‘pregnancy makes you lose your brain,’ as we don’t believe this is the case,” said Elseline Hoekzema, a researcher at Leiden University in the Netherlands, who led the study at the Universitat Autonoma de Barcelona in Spain.  “Gray matter volume loss does not necessarily represent a bad thing,” she said. “It can also represent a beneficial process of maturation or specialization.”  Pregnancy, she explained, may help a woman’s brain specialize in “a mother’s ability to recognize the needs of her infant, to recognize social threats or to promote mother-infant bonding.”  The study, which took more than five years, involved 25 women in their 30s in Spain who had never been pregnant but were hoping to conceive. Their brains were scanned before becoming pregnant and within few months after giving birth. For comparison, 20 women who had never been pregnant were also scanned twice, about the same number of months apart.  Only the pregnant women showed gray matter reduction, thinning and changes in the surface area of the cortex in areas related to social cognition. Changes were so clear that imaging results alone could indicate which women had been pregnant. The researchers said they did not yet know what was being reduced in size: neurons, other brain cells, synapses or parts of the circulatory system.  Many of the women had been recruited for the study at a fertility clinic, and the 16 who conceived after fertility treatment were compared with nine who conceived naturally. The treatments caused no difference in brain changes; nor did the sex of the babies.  The researchers also scanned the brains of 17 men who were not fathers and 19 first-time fathers before and after their partners’ pregnancies. The two male groups showed no difference in brain volume.  Researchers wanted to see if the women’s brain changes affected anything related to mothering. They found that relevant brain regions in mothers showed more activity when women looked at photos of their own babies than with photos of other children.  Six months after giving birth, the mothers answered questions on the Maternal Postnatal Attachment Scale, used to assess a woman’s emotional attachment, pleasure and hostility toward her baby. The degree of changes in the mothers’ gray matter volume predicted the degree of hostility and attachment, Dr. Hoekzema said.  Experts said more research was required, involving more women and clearer assessments of social cognition to substantiate whether gray matter loss is truly linked to “theory of mind” and improved mothering skills.  But there are some precedents for making that connection. A 2014 study showed “people with better spontaneous ‘theory of mind’ also have less gray matter volume in pretty much exactly these regions,” said Rebecca Saxe, a professor of cognitive neuroscience at the Massachusetts Institute of Technology, who was not involved in the new research.  During another period of roiling hormonal change — adolescence — gray matter decreases in several brain regions that are believed to provide fine-tuning for the social, emotional and cognitive territory of being a teenager.  “We think it is creating plasticity for learning, not that adolescent brains are wacko or making them crazy,” said Dr. Dahl of the University of California, Berkeley. Perhaps there is “a similar turbulent period around pregnancy,” he said.  In the study, the women’s cognitive abilities were tested before and after pregnancy, and “there was no loss of memory, verbal skills or working memory,” Dr. Saxe noted, providing “evidence against the common myth of ‘mommy brain.’ ”  Two years after they gave birth, scans of the brains of 11 women who had not had second children showed the same gray matter loss in the same areas, except for an area in the hippocampus, which had regained volume. Dr. Thompson said it was notable that the hippocampus, important in memory, appeared to recover, possibly because of all the learning and activity required of new mothers.  “That boost in the memory system is something that many of us in neuroscience would give our eyeteeth to achieve,” he said. Brain areas lose volume “like the erosion of the coast, but there are not many things that put the coast back.”  Many questions remain. Pilyoung Kim, an assistant professor of developmental psychology at the University of Denver, who was not involved in the study, said her research had found that some brain regions increased in size in the months after giving birth. She said she wondered if maternal brain areas waxed and waned during and after pregnancy.  Dr. Hoekzema is continuing the research, including in one very personal way. “I was pregnant with my first child when analyzing these data, but unfortunately I couldn’t get the before and after M.R.I. scans of my first pregnancy,” she said.  Now, she is 20 weeks pregnant, with her second child. “Yes, I’ve certainly scanned myself before getting pregnant,” she said, “and will go into the scanner again after birth!”  A version of this article appears in print on December 20, 2016, on Page A13 of the New York edition with the headline: Pregnancy Changes the Brain in Ways That May Help Mothering, Study Finds. Order Reprints| Today's Paper|Subscribe
The changes affect areas used in perceiving the feelings and perspectives of others, according to a new study, and may aid in recognizing an infant’s needs.