The condition known as “baby brain” may be caused by hormonal changes before birth, rather than, as commonly thought, as a result of the impact of bearing a child, scientists have said.

A study of mice by a team from the Francis Crick Institute, found their brains began preparing for “the big life change” of having children long before giving birth.

Female mice were exposed to mice pups at regular intervals before, during and after pregnancy, and researchers scored their behaviour.

Mice who were in the late stages of pregnancy demonstrated greater parental behaviour, whether they were exposed to pups or not.

The team discovered this was down to the impact of the female sex hormones, oestrogen and progesterone, on neurons in the medial preoptic area (MPOA), a part of the brain associated with parenting.

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

Previously it was believed hormones released after birth caused mothers to spend most of their time with their young.

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Virgin female rodents, the team said, are known to not interact much with pups.

Jonny Kohl, group leader of the institute’s State-Dependent Neural Processing Laboratory, pointed to changes such as the production of milk, which starts long before giving birth, and said: “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 priorities.

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

Backing up this theory, the team found that by making the neurons in the brain of mice insensitive to hormone changes, the animals no longer started to behave like parents.

And, even after giving birth, those mice didn’t show parental tendencies.

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.

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The research team – whose findings have been published in Science – thinks the brain may be re-wired 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”.

Some mothers describe being deeply affected by ‘baby brain’, which also known as ‘pregnesia’ and, according to an article on the British Psychological Society’s website is related to the widely held belief that women experience deficits in memory and cognition as a result of pregnancy.

Some men are also thought to suffer similar effects.

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