The increase in women opting to have fewer children or waiting longer to conceive could explain why multiple sclerosis (MS) is on the rise.
A study by the Murdoch Childrens Research Institute in Victoria, published on Friday, found women with at least one child have about half the risk of early MS symptoms compared to women without children.
The risk of developing symptoms of the incurable disease appear to drop with each additional child.
For example, women with three children have a 75 per cent lower risk of early MS symptoms compared to women without children.
Lead researcher Anne-Louise Ponsonby says although the study found only an association between pregnancy and a lower risk of MS symptoms - not a direct cause-and-effect link - it could help explain why the incidence of MS in women has increased in past few decades.