The new study, published today in Annals of Internal Medicine, analysed data from 15 prospective studies from around the world – including the Breast Cancer Now Generations Study.

Researchers at the University of North Carolina Lineberger Comprehensive Cancer Center, in partnership with Breast Cancer Now-funded scientists from The Institute of Cancer Research, London, analysed data from a total of 889,944 women. The team found that mothers below age 55 had a higher risk of developing breast cancer immediately after childbirth than women of the same age who hadn’t had children. 

However, the study also confirmed that in the long term, having children decreases a woman’s risk of developing breast cancer – pointing towards an overall protective effect.

Breast cancer risk initially increased, peaking at an 80% increase five years after giving birth. However, as breast cancer risk for this age group is low, the absolute risk for these women remains small. 34 years after giving birth, the mothers’ chances of developing breast cancer were 23% lower than women without children of the same age – with the crossover from increased risk to decreased risk happening around 24 years after birth.

Looking at breast cancer risk by oestrogen receptor (ER) status, the team found that whilst ER-positive breast cancer followed the overall trend, ER-negative forms of the disease did not, warranting further investigation into the mechanisms by which childbirth exerts its effects on risk on ER-negative subtypes of breast cancer.

Baroness Delyth Morgan, Chief Executive at Breast Cancer Now, which helped fund the study, said:

These are really significant findings that tell us even more about the complex effect that having children has on breast cancer risk.

Whilst having children lowers a woman’s risk of breast cancer in the long term, this study confirms there is a short-term increase in risk for a period immediately after giving birth, which peaks during the first five years.

But we’d urge new mums and mums-to-be not to be unduly concerned, and instead to know the signs and symptoms and to check their breasts regularly. It’s important to remember that younger women are at a much lower risk of breast cancer, and so even during this period of increased risk, their overall risk remains low. The longer term protective effect of having children continues well beyond the menopause – when breast cancer is more likely.

Whilst it’s normal for breasts to change throughout pregnancy and during breastfeeding, new mums and mums-to-be should check their breasts regularly and report anything they are worried about to their GP.

On the potential mechanisms behind the effects, Baroness Morgan added:

Further research remains vital to understand the mechanisms for these effects, which are likely to involve many factors. We know that a woman’s breasts become fully developed during pregnancy, with mammary cells previously in an ‘immature’ state becoming specialised, making them less susceptible to further mutations.

But, in the short term, changes to the breast during pregnancy that encourage cells to multiply may lead to any cells carrying faults that have been previously accumulated becoming cancerous. In addition, pregnancy reduces the number of menstrual cycles a woman has and so reduces her lifetime exposure to hormones like oestrogen, which drives the growth of the majority of breast tumours.

While further studies are needed to uncover the full mechanisms, we hope that confirmation of these effects could potentially lead to new ways to prevent breast cancer in the future.

The Breast Cancer Now Generations Study is a landmark prospective study of the causes of breast cancer that is following over 113,000 UK women for 40 years. With more women being diagnosed with breast cancer than ever before, the Study – which is based at The Institute of Cancer Research (ICR) – continues to investigate the genetic, lifestyle and environmental factors that may change a woman’s risk of developing breast cancer.

The Study has already led to a number of significant discoveries into the interlinked causes of breast cancer, including clarifying that women taking combined HRT are 2.7 times more likely to develop breast cancer than non-users, and that smoking is particularly associated with an increased risk of breast cancer among those who began smoking during adolescence.

Breast Cancer Now thanks M&S for their generous support of the Breast Cancer Now Generations Study – as well as The Liz and Terry Bramall Foundation for helping make the long-term study possible.