Investigators have found that the risk of developing breast cancer could be nearly three times higher among women who experience notable weight gain after the age of 20 years and either give birth after age 30 or don’t have children compared with those who give birth before age 30 and whose weight remains relatively stable, according to new findings presented by Malcomson et al at the European Congress on Obesity (ECO) 2025.
Background
Breast cancer is one of the most common cancer types globally, with 2.3 million diagnoses and 670,000 deaths among women in 2022.
Previous research has shown that weight gain in adulthood could increase the risk of developing breast cancer following menopause. Further studies have uncovered that an early first pregnancy may reduce the risk of breast cancer. For instance, a review of 21 studies focused on reproductive risk factors for breast cancer reported that each additional year of age of first pregnancy could add 5% to the risk of premenopausal breast cancer and 3% to the risk of postmenopausal breast cancer. However, investigators have yet to establish whether the combined effect of these two factors and whether weight gain could have less effect on breast cancer risk if a woman has an early first pregnancy.
“[T]he proportion of the women with obesity or overweight has increased … and the proportion of women giving birth later in life has been steadily increasing over the past 50 years”, detailed lead study author Lee Malcomson, BSc, of the University of Manchester. “Meanwhile, diagnoses of breast cancer in women are at their highest ever rate. More information about how age at first pregnancy 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,” he added.
Study Methods and Results
In the study, the investigators examined the data of 48,417 women with a median age of 57 years and a median body mass index of 26.3 kg/m² who participated in the PROCAS study—which involved women attending breast screening.
The investigators categorized the women based on whether they had their first pregnancy early (prior to the age of 30 years) or late (after age 30 years), whether they were nulliparous, as well as their weight gain in adulthood. Weight gain was calculated by asking the participants to recall their weight at the age of 20 years and subtracting it from their current weight.
After a median follow-up of 6.4 years, 1,702 were diagnosed with breast cancer. The investigators revealed that women with an early first pregnancy had a greater weight gain during adulthood compared with those with a late first pregnancy, with 0.21 kg of additional weight gain for each year earlier the pregnancy occurred. They also confirmed prior findings that an early first pregnancy could protect against postmenopausal breast cancer and that adult weight gain may be associated with an increased risk of the disease.
The investigators discovered no evidence that having a first pregnancy at an early age reduced the increased breast cancer risk caused by weight gain. However, they determined that women who had over a 30% increase in weight during adulthood and either had their first child after age 30 years or didn’t have children were 2.73 times more likely to develop breast cancer compared with women who had an early first pregnancy and less than a 5% increase in adult weight.
Subanalyses including women with estrogen receptor–positive breast cancer, ductal carcinoma in situ, breast cancer detected during screening, and postmenopausal participants had similar results.
Conclusions
The findings aligned with advice to keep a healthy weight and exercise to minimize the risk of developing breast cancer.
“Our study is [among] the first to establish how weight gain and age of first birth interact to affect a woman’s risk of breast cancer,” Mr. Malcomson indicated. “It is vital that [physicians] are aware that the combination of gaining a significant amount of weight and having late first birth—or … not having children—greatly increases a woman’s risk of the disease,” he concluded.
Disclosure: For full disclosures of the study authors, visit eco2025.org.
