If you were to do a little time traveling, flashing back to when you were 18, how would your weight differ from what it is today? According to research by the American Cancer Society, the amount of weight a woman gains after the age of 18 is a strong indicator as to whether she will get breast cancer later in life.

Weight gain and body mass were identified long ago as risk factors for breast cancer. The Cancer Society estimates between one-third and one-half of all breast cancer deaths among older women have been contributed to weight.

Fat tissue makes estrogen, and estrogen can help breast cancer grow. Heather Spencer Feigelson, senior epidemiologist with the American Cancer Society said, “Breast cancer is strongly dependent on body weight. Even modest amounts of weight gain lead to a significantly increased risk of breast cancer.”

One of the largest studies of breast cancer and weight included 1,934 breast cancer cases among 62,756 women involved in a separate long-term study. Women ages 50 to 74, who were post-menopausal, were asked their weight in 1992 when the study began and their weight when they were 18 years old. They were also sent questionnaires at yearly intervals.

The researchers said older women who gained 20 to 30 pounds after high school graduation were 40 percent more likely to get breast cancer than women who kept the weight off. If the weight gain was more than 70 pounds, the risk was doubled. Lean post-menopausal women not taking hormone replacement therapy produce very little estrogen and had the lowest cancer risk in the study.

Associate professor of surgery at Columbia University, Dr. Paul Tartter said, “The more fat you have—fat cells are capable of synthesizing estrogen—the heavier you are, the higher your estrogen levels. There’s no question that estrogen is the common denominator of most of our risk factors for breast cancer.”

A study published in the Archives of Internal Medicine suggests that weight gain throughout adulthood is linked to an increased risk of developing breast cancer after menopause for women who do not undergo menopausal hormone therapy. Scientists already knew that being overweight or obese increased breast cancer risks after menopause, but the correlation between the timing and the amount of weight gain in adulthood had not been investigated before.

The study showed the importance of the timing of when the weight gain occurred. Women who put on more than 66 pounds between the ages of 18 and 35 had a 65 percent higher risk, but those whose weight increased by the same amount between 35 to 50 had a 223 percent higher risk of developing breast cancer after menopause.

The results of these studies appear to confirm that putting more estrogen into the body increases the risk of breast cancer for older women, whether the estrogen comes from hormone therapy or from being overweight. The postmenopausal women at lowest risk seem to be the ones with the lowest levels of estrogen, who do not use menopausal hormone therapy, and who stay lean.