Breast cancer: Who is at risk?
If they have a baby for the last time between the ages of 35 and 39, the risk peaks six years later - when they are 40% more likely to be diagnosed than women with no children.
Women who become mothers for the last time aged 25 to 34 have an increased risk of 25%, peaking five years after their final child. Under-25s have no additional breast cancer risk.
Scientists said the older a woman is when she has her last child, the higher the risk rises.
Experts have always believed that having a child protects against breast cancer. But the new study found the risk actually rises for about two decades after childbirth before the protective effect kicks in.
The average age at which women give birth in Britain has soared in the past four decades, rising to 30.4 in 2016 from 26.4 in 1974, according to the Office for National Statistics.
Scientists behind the study stressed that the cancer risk associated with childbirth eventually drops again. They said in the long-term motherhood reduces breast cancer risk.
But their findings, based on 900000 people across 15 other studies, suggests women are at increased risk for about 24 years after the birth of their last child.
The new study was compiled by the Institute of Cancer Research in London and the University of North Carolina in the US.
Dr Hazel Nichols, of North Carolina, said: “What most people know is that women who have children tend to have lower breast cancer risk than women who have not had children, but that really comes from what breast cancer looks like for women in their 60s and beyond.
“We found that it can take more than 20 years for childbirth to become protective for breast cancer, and that before that, breast cancer risk was higher in women who had recently had a child.”
The team found that the risk increased further for women who had more than one child, with those who have three or more twice as likely after three years to be diagnosed with breast cancer than women with no children.
Scientists believe the increased risk is caused by the changes in breasts during pregnancy - when a surge of hormones prepare for breastfeeding.
Dr Minouk Schoemaker, of the Institute of Cancer Research, said these changes sometimes force cells to mutate, forming tumours. This is more likely in older women, whose cells have already been through several changes.
Baroness Delyth Morgan of Breast Cancer Now, said: “These are really significant findings that tell us even more about the complex effect that having children has on breast cancer risk.”
Breast cancer is Britain’s most common female cancer, with 55000 new cases and 11500 deaths every year.
In South Africa, it affects about 27 in 100000 women, and accounts for 16% of cancer deaths among women.