The surge in hormones women experience during the nine months of pregnancy can influence the development of the central nervous system, explains neuroscientist, Liisa Galea from University British Columbia in Canada, who has conducted a number of studies into how pregnancy changes a woman's brain chemistry.
"The more children you have given birth to, the greater risk you face of getting dementia in later life," Galea said at the 9th Annual Canadian Neuroscience Meeting, while talking about a recent animal study investigating the effects of pregnancy hormones.
Galea and her researchers looked at two hormones used to treat the symptoms of menopausal women and found they have different effects depending on the age of the subject and if they had given birth.
They also found that surge of oestrogen during pregnancy alters levels of neuroplasticity in the brain which is responsible for memory and spatial awareness.
In an interview recorded at the university where she works, Galea stated that some changes in the brain are temporary, but others are permanent.
"Our research shows that, as a result of these transformations, mothers experience different cognitive abilities and health risks than women without children," she said. "They may even react to medication differently."
When an oestrogen hormone called oestrone was given to middle-age female rats that had already experience motherhood, it impaired the ability to learn and memorise.
However, when it was given to female rats of the same age that had not previously given birth, it improved learning and memory – suggesting that prior pregnancy had affected the brain permanently.
As oestrone is a component of some of the most common forms of hormone replacement therapy, these findings could have implications for the treatment of age-related neurodegenerative disorders in women, the researchers said.
“If you have given birth before, you have a better memory but experience an increased risk of developing Alzheimer’s like conditions. But we don’t know much about why this is the case,” Dr Galea told The Independent.
"More children are probably enriching, but they are also probably more stressful,” Dr Galea added.