MONDAY, May 12, 2025 (HealthDay News) — Weight gain combined with late first pregnancy (FP) or nulliparity is associated with an increased risk for breast cancer (BC), according to a study presented at the annual meeting of the European Congress on Obesity, hosted by the European Association for the Study of Obesity from May 11 to 14 in Malaga, Spain.

Lee Malcomson, from the University of Manchester in the United Kingdom, and colleagues examined the effect of parity and adult weight gain on BC risk using data from 48,417 women. Using incident BC as the primary outcome, associations were explored across four groups: first pregnancy (FP) at younger than 30 years, low weight gain (<15 percent); FP at younger than 30 years, high weight gain (≥15 percent); nulliparous or FP at 30 years or older, low weight gain; and nulliparous or FP at 30 years or older, high weight gain.

The researchers identified 1,702 new BCs during a median 6.4 years of follow-up. Compared with those with a late FP, those with an earlier FP had a greater weight gain during adulthood (β, −0.39). The risk for developing BC was significantly higher for those who were nulliparous or had FP at 30 years or older with high weight gain compared with those with an early FP with low weight gain (hazard ratio, 2.73). Similar results were seen in sensitivity analyses for estrogen receptor-positive BC, ductal carcinoma in situ, screen-detected BC, and postmenopausal participants.

“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,” Malcomson said in a statement.

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