The neuroimaging study, conducted in Spain, was prospective, looking at the brains of 25 first-time mothers before and after pregnancy, and again two years after the women gave birth. The researchers compared the brain images of these new mothers with those of 19 first-time fathers, as well as 17 men and 20 women without children. The pattern of structural changes the researchers observed in the new mothers were so distinct that it was possible to identify the mothers just from their brain scans. Those changes endured for at least two years, except for a partial return to its previous state in the hippocampus, a brain structure heavily involved with memory.

The MRI study showed changes in gray matter, the outer layer of the brain that contains the cell bodies of neurons. The gray matter in certain areas shrunk in size after pregnancy, a phenomenon known as “gray matter pruning.” A similar shrinkage is seen in early childhood and during adolescence. The gray matter contains many interconnections among neurons, and during pruning, the most important connections are strengthened and the others are left to wither. Rather than indicating a loss of ability, pruning is generally taken to mean that a brain region has become more specialized.

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The researchers also found that some women had more gray matter pruning than others, and those with the most pruning seemed to bond best with their babies. “The gray matter volume changes of pregnancy significantly predicted the quality of mother-to-infant attachment and the absence of hostility toward their newborns in the postpartum period,” the authors wrote in a study published Monday in Nature Neuroscience.

In a further experiment, the researchers showed women pictures of several babies and found, unsurprisingly, that the women's brains responded more strongly to photos of their own babies. The brain images, they said, revealed “the strongest neural activity in response to the women's babies corresponded to regions that lost gray matter volume across pregnancy.”

Pregnancy is associated with a surge of sex hormones akin to the heightened production of sex hormones during puberty. The researchers noted that gray matter also is pruned during adolescence, when a spectrum of emotional, cognitive and behavioral neural changes begin to fine tune the teenage brain.

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Cordelia Fine, a psychologist at the University of Melbourne who had no part in this research, said that “this is a solid study,” despite the small sample size. Fine has written several books about gender and how male-female differences are often overstated in science. “The authors are appropriately careful not to conclude that the brain changes they observe are caused by hormones (or by hormones alone), since they don’t show this directly. However, this is certainly a plausible hypothesis.”

Fine cautions that the brain areas identified by the scientists are also responsible for other functions, not simply attachment. A more significant caveat, she said, is that while the authors of the study “speculate that the structural brain changes they see underlie maturation” of neural networks that “could facilitate attachment . . . it's worth noting that the authors measured empathy before and after pregnancy, and found no change.”

According to the authors of the study, these brain changes may “serve an adaptive purpose for pending motherhood,” that is, the stronger the mother-child attachment, the greater chance the child survives.