Significant weight gain paired with delayed or foregone motherhood nearly triples a young woman’s risk of later breast cancer, a new study says.

Women were 2.7 times more likely to develop breast cancer if they:

• Had a more than 30% increase in weight after age 20.

• Either delayed their first child until after age 30 or didn't have children at all.

“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,” lead researcher Lee Malcomson, a research associate at the University of Manchester in the U.K., said in a news release.

“It is vital that general practitioners 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 said.

For the study, researchers analyzed data on nearly 48,500 women participating in a large study of breast screening. The women’s average age was 57 and they had an average body mass index (BMI) of 26, which is considered overweight. BMI is an estimate of body fat based on height and weight.

Researchers asked them to recall their weight at 20, and calculated their weight gain from then based on their current weight. 

The women were followed for about six years on average, and during that period more than 1,700 were diagnosed with breast cancer.

The results showed that an early first pregnancy protects against post-menopausal breast cancer, even though women are more likely to gain more weight with an earlier pregnancy.

But gaining weight while delaying pregnancy increases the risk of later breast cancer, researchers found.

“In England, the proportion of the women with obesity or overweight has increased from 49% in 1993 to 59% in 2021, and the proportion of women giving birth later in life has been steadily increasing over the past 50 years,” Malcomson said.

“Meanwhile, diagnoses of breast cancer in women are at their highest ever rate,” he continued. “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.”

Researchers presented these findings Saturday at the European Congress on Obesity in Malaga, Spain.

Findings presented at medical meetings should be considered preliminary until published in a peer-reviewed journal.