Pregnancy: hormone changes may lead to permanent rewiring of the brain, researchers suggest

Pregnancy could lead to a permanent rewiring of neurons, as hormones change the brain during pregnancy which could explain what is often referred to as “baby brain”, researchers have suggested.

The research, in mice, revealed their parenting instincts were triggered by changes in the brain that occur in response to oestrogen and progesterone late in pregnancy. In the study, the researchers exposed female mice to mice pups at regular intervals before, during and after pregnancy, scoring their behaviour.

A team from the Francis Crick Institute said their discovery shows the brain is preparing for the “big life change” of having children long before the actual birth.

Previously, it was thought hormones released after birth were responsible for the change, although earlier studies showed rats who had given birth by Caesarean section, and virgin mice exposed to pregnancy hormones, still displayed maternal behaviour.

The team found an increase in parental behaviour in mice who were in the late stages of pregnancy, and exposure to pups was not necessary for the change.

Recordings showed neurons in an area of the brain associated with parenting, known as the medial preoptic area (MPOA), were impacted by female sex hormones oestrogen and progesterone.

Oestrogen simultaneously reduced the baseline activity of these neurons and made them more excitable, the researchers said, while progesterone rewired their inputs.

Jonny Kohl, group leader of the State-Dependent Neural Processing Laboratory, said: “We know that the female body changes during pregnancy to prepare for bringing up young.

“One example is the production of milk, which starts long before giving birth. Our research shows that such preparations are taking place in the brain, too.”

Mr Kohl said the changes – often referred to as “baby brain” – cause a change in priority.

“Virgin mice focus on mating, so don’t need to respond to other females’ pups, whereas mothers need to perform robust parental behaviour to ensure pup survival,” he added.

“What’s fascinating is that this switch doesn’t happen at birth – the brain is preparing much earlier for this big life change.”

Making the neurons in the brain of mice insensitive to hormone changes removed the onset of parental behaviour, with mice in the study failing to show parental tendencies even after birth.

Researchers said the discovery shows there is a “critical period during pregnancy when these hormones take effect”.

Some changes lasted for “at least” a month after birth, with some changes permanent, suggesting pregnancy can lead to the long-term rewiring of the female brain.

The research team – whose findings have been published in Science – thinks the brain may be rewired in a similar way during pregnancy in humans, with the same hormonal changes thought to have an impact across similar areas of the brain.

They said this could “influence parental behaviour alongside environmental and social cues”.

Rachida Ammari, postdoctoral fellow at the Francis Crick Institute, and first author along with PhD student Francesco Monaca, said: “We’ve demonstrated that there’s a window of plasticity in the brain to prepare for future behavioural challenges.

“These neurons receive a large number of inputs from elsewhere in the brain, so now we’re hoping to understand where this new information comes from.”