Having a child may raise a woman's odds of getting breast cancer - and those who wait until they are 35 to start a family face the biggest risk, a major study found.
Mothers face an 80 per cent greater chance of being struck down with the disease than childless women of the same age, five years after giving birth.
Researchers at the University of North Carolina warned those who have a baby after turning 35 face the greatest risk - but there is no higher odds for under-25s.
Giving birth - especially more than once - has long been thought to protect women against breast cancer by making changes to mammary gland cells that make them less susceptible to cancer.
But the relationship between age, childbearing and breast cancer is a complicated one.
The new study, of nearly 900,000 women, found the risk actually rises for about two decades after childbirth before the protective effect kicks in.
Breast cancer is second to only skin cancer as the most commonly diagnosed form of the disease in women.
About one out of every eight women will have breast cancer at some point in her life.
It's a very real possibility that women live in fear of, and may even shape major life decisions, as it did for Angelina Jolie, who had preventative double mastectomies after discovering she carried the BRCA1 gene in 2013.
The gene is hardly a pure predictor of breast cancer - and none of the other risk factors for the disease.
Older age is another key factor, but the age a woman is when she has her first child and the number of children she has also must enter the calculation of breast cancer risks.
And that calculation is perhaps more complicated than we once thought, as confirmed by the new research.