Women who have gained more than 20 pounds since age 18 have a higher risk of developing breast cancer after menopause than women who have kept their girlish figure. The finding was published by American Cancer Society researchers in the journal Cancer Epidemiology Biomarkers and Prevention (Vol. 13, No. 2: 220-224).

"These findings further illustrate the importance of maintaining a healthy body weight throughout life," said lead researcher Heather Spencer Feigelson, PhD, MPH. "Even modest weight gain … was associated with increased risk of postmenopausal breast cancer."

Researchers have long known that being overweight increases the likelihood that a woman will develop breast cancer. But the new study puts the risk into very real terms: pounds on the scale.

ACS researchers questioned more than 62,000 postmenopausal women about their height, current weight, weight at age 18, and use of hormone replacement therapy (HRT). After accounting for other breast cancer risk factors -- like family history of the disease, exercise, and alcohol use -- they found that women who had gained 21-30 pounds since age 18 had a 40% higher risk of breast cancer than women who'd stayed within 5 pounds of their teenage weight. Those who had gained more than 70 pounds had double the risk. (Lesser weight gain, between 6-20 pounds, had little effect on risk, less than 10%.)

The findings applied only to women who had never used HRT or who were no longer using it. Among women who were using HRT at the time of the study, weight gain did not appear to influence risk, probably because HRT "trumps" weight as a factor in breast cancer development. Both HRT and excess fat raise estrogen levels, which increases the risk of breast cancer. But the researchers said HRT raises estrogen so much that even a lot of extra weight has only a minimal effect by comparison.

Fortunately, there are steps women can take to lower their risk of breast cancer. In addition to maintaining a healthy body weight, the ACS also recommends staying physically active, limiting alcohol consumption, and staying away from hormone therapy, if possible. Regular screenings are also important, since they can detect breast tumors when they are small and more easily treated.