Having children is known to have a complex effect on breast cancer risk. Overall, in the long term, pregnancy reduces the risk of the disease for most women. But in the shorter term, studies suggest that a woman’s risk slightly increases after giving birth – an increase that is temporary, lasting a number of years.

The age at which a women has children is known to affect risk (the earlier a woman begins her family, the lower her breast cancer risk) as is the number of children you have (the more children a woman has, the lower her risk) – thought to be due to the changes in hormone levels and biological changes happening in the breast during pregnancy. But, the role of some other key characteristics of pregnancy in breast cancer risk is still uncertain, with evidence remaining inconsistent to date.

In a new analysis of 83,451 UK women from the Breast Cancer Now Generations Study who had given birth, scientists investigated the effect of length of pregnancy and birthweight of a woman’s first child on breast cancer risk, finding that a short pregnancy (26-31 weeks) was associated with an increased risk, as was having a heavier first-born (over 4,500g).

Through questionnaires at recruitment to the study and during follow-up, scientists at The Institute of Cancer Research, London, established detailed information on each pregnancy that participants had had as well as a wide range of variables known to be linked to breast cancer. 

Further data were collected at approximately three-years, six-years and nine-years to enable the researchers to adjust – unlike in previous studies – for a full range of non-reproductive as well as reproductive risk factors, such as alcohol, smoking or breastfeeding, that may confound the results.

The study, published in Breast Cancer Research, observed that 1,767 out of the 83,451 went on to develop the disease, finding that women whose firstborns were heavier than 4,500g (9lb 14.7 oz) were 53% more likely to go on to develop breast cancer than those whose firstborns weighed between 3,000g and 3,499g.

It should be noted that this 53% relative risk increase was observed with 95% confidence intervals of 6% to 121% – so while the study found a statistically significant increase in risk, the exact size of the increase could still fall within a fairly wide range and needs to be further established.

The study also demonstrated an inverse trend between length of gestation and the risk of breast cancer, with those who had given birth between 26-31 weeks being 30% more likely to develop the disease in their lifetime than those who had given birth at 40-41 weeks. The effect was stronger specifically for premenopausal breast cancer, with those who had given birth between 26-31 weeks observed to be nearly 2.4 times more likely to develop breast cancer before the menopause than those who’d given birth at 40-41 weeks.

Breast cells are known to proliferate (grow and divide quickly) in the first and second trimesters of pregnancy, and differentiate (mature and become more specialised cells) in the third trimester. The authors therefore suggest that a pregnancy that ends at an earlier stage might lead to increased risk through fast cell growth and high sex hormones levels promoting growth of partially transformed cells, without subsequent differentiation.

Regarding birthweight, women giving birth to heavier offspring tend to have higher levels of oestrogen and a protein called IGF1 (insulin like growth factor 1) in pregnancy, both of which have been associated with breast cancer development, giving a plausible mechanism for the link which further studies would need to investigate. 

Breast Cancer Now has today called for further research to investigate the findings – but with these factors neither clear-cut nor things women can change, the charity has urged women not to be concerned but instead to take healthy lifestyle steps to help keep their risk as low as possible. 

The study was led by Dr Michael Jones and Professor Anthony Swerdlow, from The Institute of Cancer Research, London – and funded by Breast Cancer Now.

Professor Anthony Swerdlow, Professor of Epidemiology at The Institute of Cancer Research, London, said:

Many lifestyle, genetic and other factors together contribute to a woman’s risk of breast cancer, and having children is known to have a complex effect on breast cancer risk.

In our analysis of a large cohort of women who had given birth, we found that both a shorter pregnancy and a higher birthweight in a woman’s firstborn child raised her risk of developing breast cancer.

Further studies are needed to estimate more precisely the extent to which a shorter pregnancy and heavier firstborns increase women’s risk of developing breast cancer, and to study the possible causal mechanism in more depth.

Baroness Delyth Morgan, Chief Executive at Breast Cancer Now, which funded the research, said:

These are important findings because we can now go on to investigate why these aspects of pregnancy may increase breast cancer risk.

Our risk is affected by a combination of genes, lifestyle and environment. With several contributing factors relating to having children, it’s essential that we continue to untangle the complex links between pregnancy, female sex hormones and breast cancer risk.

But we’d urge women not to be unduly concerned by these results, which now need further exploration. Having a significantly early birth is not something that women can change, and there is also no straightforward way to influence birthweight, which is determined by several factors.

While there are some factors you can’t change, there are healthy lifestyle steps all women can take both before and after having children to keep their risk of breast cancer as low as possible – including keeping physically active and limiting alcohol intake.”