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 DrHoekzema 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 DrHoekzema’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 DrHoekzema 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.