A new study from the UK, presented at the European Congress on Obesity (ECO 2025), reveals that women who gain significant weight after age 20 and have their first child after 30—or do not have children at all—are almost three times more likely to develop breast cancer compared to those who give birth earlier and maintain a stable weight. The findings shed new light on how these two factors interact to influence breast cancer risk.
Breast cancer remains one of the most common cancers worldwide, with 2.3 million women diagnosed and 670,000 deaths recorded in 2022. In the UK alone, nearly 57,000 cases are diagnosed each year, resulting in 11,500 deaths. According to Lee Malcomson, the study’s lead researcher from the University of Manchester, the number of women diagnosed with breast cancer has reached record highs in recent years.
He noted that this increase coincides with rising obesity rates and more women having children later in life. The percentage of women who are overweight or obese in England grew from 49% in 1993 to 59% in 2021, and the age of first-time mothers has steadily increased over the past 50 years.
Previous research has established that weight gain in adulthood heightens the risk of developing breast cancer after menopause. Other studies have shown that having a first child at an earlier age can reduce the risk.
For instance, a review of 21 studies found that each additional year of age at first full-term pregnancy increases the risk of premenopausal breast cancer by 5% and postmenopausal breast cancer by 3%. However, the combined effect of weight gain and late motherhood on breast cancer risk had not been clearly understood—until now.
To explore this connection, Malcomson and his colleagues analyzed data from 48,417 women with a median age of 57 and an average BMI of 26.3 kg/m². These participants were part of the PROCAS study, which tracks women attending breast cancer screenings in the UK.
The researchers categorized the women based on the age at which they had their first child: early (before 30), late (30 or older), or never (nulliparous). They also measured weight gain by comparing each woman’s weight at age 20 to her current weight. Over a median follow-up period of 6.4 years, 1,702 of these women were diagnosed with breast cancer.
The results were revealing. Women who had their first child at an early age tended to gain more weight during adulthood compared to those who had children later. Specifically, each year earlier a woman gave birth was linked to 0.21 kg of additional weight gain.
Despite this, the study confirmed that early motherhood does provide some protection against postmenopausal breast cancer, while significant weight gain increases the risk of the disease.
Importantly, the study found no evidence that having a first pregnancy at an early age reduces the breast cancer risk associated with adult weight gain.
Instead, it showed that women who gained more than 30% of their body weight after age 20 and either had their first child after 30 or did not have children were nearly three times more likely (2.73 times) to develop breast cancer than women who gave birth earlier and kept their weight gain under 5%.
Malcomson emphasized the importance of these findings, stating, “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.” He stressed that general practitioners (GPs) should be aware of this combination of factors, as it significantly increases the risk of breast cancer.
The study’s findings also support existing public health advice: maintaining a healthy weight and exercising regularly are crucial for reducing breast cancer risk. The researchers noted that these lifestyle choices are particularly important for women who plan to have children later in life or do not plan to have children at all.
The study also included sub-analyses focusing on women with ER-positive breast cancer—the most common form of the disease—as well as those with ductal carcinoma in situ (DCIS), which is an early form of breast cancer.
Women who were diagnosed during routine screening and those who were postmenopausal showed similar risk patterns, reinforcing the connection between late motherhood, weight gain, and breast cancer.
This research provides crucial insights into breast cancer risk factors and highlights the need for women and healthcare providers to be mindful of the combined effects of weight gain and delayed childbirth. With further awareness and preventive strategies, it may be possible to lower breast cancer rates among at-risk women.
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