Women who had their first child after 35 may be at an increased risk of developing breast cancer than their peers who do not have children, according to a study contrary to conventional wisdom that childbirth is protective against breast cancer.

Besides late childbirth, women who had a family history of breast cancer or who had a greater number of births also had an increased risk for breast cancer after childbirth. The pattern looked the same whether or not women breastfed.

While the risk was higher for women who were older at first birth, there was no increased risk of breast cancer after a recent birth for women who had their first child before 25, said researchers from the University of North Carolina (UNC) in the US.

“This is evidence of the fact that just as breast cancer risk factors for young women can differ from risk factors in older women, there are different types of breast cancer, and the risk factors for developing one type versus another can differ,” said Hazel B. Nichols, Professor at the UNC.

Although childbirth is still protective against breast cancer, researchers say it can take more than two decades for benefits to emerge.

Breast cancer is more common in older women, with the median age of 62 at diagnosis. But, the study, published in the Annals of Internal Medicine, identified elevated breast cancer risk after childbirth in women younger than 55.

In women 55 years and younger, breast cancer risk peaked about five years after they gave birth, with risk for mothers 80 per cent higher compared with women who did not gave birth.

Twenty-three years after giving birth, women saw their risk level off, and pregnancy started to become protective.

For their analysis, the team pooled data from 15 prospective studies from around the globe that included 889,944 women. In addition to looking at breast cancer risk after childbirth, they also evaluated the impact of other factors such as breastfeeding and a family history of breast cancer.

The findings could be used to develop better breast cancer risk prediction models to help inform screening decisions and prevention strategies, Nichols said.