As any parent will tell you, once you have had children nothing is ever quite the same. Including, it seems, their mothers’ brains. In a paper just published in Nature Neuroscience, a team led by Elseline Hoekzema of the Universitat Autònoma de Barcelona, in Spain, describe for the first time how pregnancy alters women’s brains, rewiring them in ways that persist long after a child has been born.
Dr Hoekzema and her colleagues performed detailed brain scans on 65 female volunteers, none of whom had been pregnant before, but hoped to be in the near future, and a further 20 who had no such desire. About 15 months later, by which time 25 of their volunteers had carried babies to term, they repeated the process.
Comparing the scans showed significant reductions in the volume of grey matter in the brains of the new mothers. (Grey matter contains the main bodies of nerve cells; white matter, the brain’s other component, consists mostly of the nerve fibres that link those cells together.) The effect was reliable enough that it could be used by itself to predict, with perfect accuracy, which of the women had been pregnant and which had not. And it was persistent, too. When the researchers retested the mothers two years later, most of the alterations were still present.
Dr Hoekzema and her colleagues suspected that something in the biological process of pregnancy itself was causing the changes. To double-check, and to make sure that the experience of preparing for parenthood was not the true culprit, they also compared their women’s brains with those of some men—both fathers and those without children. The men’s brains, like those of the childless women, showed no such pattern of changes. And the results fit with studies on animals. Rats that have had pups, for instance, show notable and lasting changes in brain structure. They also tend to be less anxious, better able to cope with stress, and to have better memory than their pupless contemporaries.
Pregnancy, then, does indeed do things to a woman’s brain. But what exactly those things mean is hard to tease out. Neuroscientists do not really understand how the brain works. That makes it difficult to predict how a change in the organ’s structure will alter the way it functions.
Some of the changes took place in the hippocampi. These are a pair of small, banana-shaped structures buried deep in the brain, one in each hemisphere, that are known to be important for forming memories. Administering a few simple cognitive tests to the new mothers—including tests of memory—revealed no obvious changes in performance. And the hippocampi had partially regrown within two years. But Dr Hoekzema and her colleagues point out that most of the more permanent reductions in grey matter happened across several parts of the brain that, in other experiments, have been found to be associated with the processing of social information, and with reasoning about other people’s states of mind.
That would make sense from an evolutionary point of view. Human babies are helpless, and, in order to care for one, a mother must be good at inferring what it needs. The rewiring may also affect how well women bond with their infants. After the women in Dr Hoekzema’s study had given birth, the research team administered a standard psychological test designed to measure how attached those women had become to their babies. The ones with the greatest reductions in grey-matter volume were, on the whole, the most strongly bonded.
Ascribing all this to a reduction in grey-matter volume, rather than an increase, sounds counter-intuitive. But Dr Hoekzema reckons it is probably evidence of a process called synaptic pruning, in which little-used connections between neurons are allowed to wither away, while the most-used become stronger. That is thought to make neural circuitry more efficient, not less so.
She points out that the surge of sex hormones people experience during adolescence is thought to cause a great deal of synaptic pruning, moulding a child’s brain into an adult one. So it is reasonable to assume that the even greater hormonal surge experienced during pregnancy might have a similar effect. When it comes to the brain, after all, bigger is not necessarily better.