Women who experience significant weight gain after the age of 20 and either have their first child after the age of 30 or don't have children are almost three times more likely to develop breast cancer than those who give birth earlier and whose weight remains relatively stable, new research from the UK being presented at this year's European Congress on Obesity (ECO 2025) has found.
Previous research has shown that weight gain in adulthood increases the risk of developing breast cancer after the menopause.
Other research has found that an early first pregnancy may cut the risk of breast cancer. For example, a review of 21 studies on reproductive risk factors for breast cancer reported that each additional year of age of first full-term pregnancy adds 5% to the risk of premenopausal breast cancer and 3% to the risk of postmenopausal breast cancer.1
However, the combined effect of these two factors and whether weight gain has less effect on breast cancer risk if a woman has an early first pregnancy, has yet to be established.
Breast cancer is one of the most common cancers worldwide, with 2.3 million diagnoses in women in 2022 and 670,000 deaths.2 In the UK, where it is the most common cancer, there are almost 57,000 cases and 11,500 deaths a year.3
"In England, the proportion of the women with obesity or overweight has increased from 49% in 1993 to 59% in 20214, and the proportion of women giving birth later in life has been steadily increasing over the past 50 years5", says lead researcher Lee Malcomson, of the University of Manchester, Manchester, UK.
"Meanwhile, diagnoses of breast cancer in women are at their highest ever rate.6 More information about how age of motherhood and weight gain affect the risk of breast cancer, would allow us to better work out who is most at risk of the disease and target lifestyle advice accordingly."
For the study, Mr Malcomson and colleagues analysed data on 48,417 women (median age 57 years, median BMI 26.3 kg/m²) who took part in PROCAS7, a large UK study of women attending breast screening.
The women were categorised based on whether they had their first pregnancy early (before the age of 30 years), late (aged 30 years or older) or were nulliparous (had not given birth) and weight gain in adulthood. Weight gain was calculated by asking participants to recall their weight at the age of 20 and subtracting it from their current weight. The women were followed up for a median of 6.4 years, during which 1,702 were diagnosed with breast cancer.
Analysis of the results found that women with an early first pregnancy had greater weight gain during adulthood than those with a late first pregnancy, with 0.21kg of additional weight gain for each year earlier the pregnancy occurred.
It also confirmed the literature that an early first pregnancy protects against post-menopausal breast cancer and that adult weight gain is associated with an increased risk of the disease.
However, the study found no evidence that having a first pregnancy at an early age reduces the increased breast cancer risk caused by weight gain.
It did show that women who had a greater than 30% increase in weight during adulthood and either had their first child after age 30, or did not have children, were nearly three times more likely (2.73 times) to develop breast cancer compared with women who had an early first pregnancy and less than a 5% increase in adult weight.
Mr Malcomson concludes: "Our study is the first to establish how weight gain and age of first birth interact to affect a woman's risk of breast cancer.
"It is vital that GPs are aware that the combination of gaining a significant amount of weight and having late first birth - or, indeed, not having children - greatly increases a woman's risk of the disease."
He added that the results also align with advice to keep a healthy weight and exercise to cut the risk of breast cancer.
Sub-analyses of data on women with ER positive breast cancer (the most common form of breast cancer), ductal carcinoma in situ (DCIS – a very early form of breast cancer), breast cancer detected during screening and post-menopausal participants had similar results.
