New study ... Mums of boys have male DNA in different parts of their brains.

New study ... Mums of boys have male DNA in parts of their brains.

Rewarding though pregnancy may be, any mum can attest that carrying a baby into the world can take a toll on the body. And researchers just keep adding to the list of ways that nine months of companionship can leave a lasting health impact on mother and child.

While it's a common complaint from new mums that babies scramble their brains, a new study from the Fred Hutchinson Cancer Center in Seattle shows that being pregnant with a boy typically leaves women with a dollop of male DNA in their heads.

Women who have given birth at least once also have a lower risk for rheumatoid arthritis compared to women with no children 

Scientists have long known that genetic material and cells are exchanged between fetus and mother during pregnancy, but the study is the first to show that fetal cells can cross the human blood/brain barrier.

The study, published in the journal PLOS One, reviewed brain autopsy specimens from 59 women who had died between the ages of 32 and 101. Male cells were detected in 63 per cent of the subjects, in multiple brain regions.

The effects of the male DNA in a female brain are largely unknown but very early research suggests that it may protect against Alzheimer's disease, as the study found that the women who didn't have male DNA were more at risk for the degenerative disease than the others.

Other studies at Hutchinson and elsewhere have linked the swaps between fetus and mother to a greater or lesser risk of developing some types of cancer and autoimmune disease.

For example, cells of fetal origin are thought to offer some protection against breast cancer, while they're associated with an increased risk of colon cancer. And women who have given birth at least once have a lower risk for rheumatoid arthritis compared to women with no children.