Breast cancer risk linked with fertility timing

NEW YORK (Reuters Health) - A longer interval between the age a woman first begins to menstruate and her age when she first gives birth is associated with an increased risk of breast cancer, the results of a study published in the American Journal of Epidemiology suggests.

Age at menstruation and first birth are "established risk factors for breast cancer," Dr. Christopher I. Li, of the Fred Hutchinson Cancer Research Center, Seattle, and colleagues write. The interval between these ages may also influence breast cancer risk because the breast becomes more susceptible to carcinogenic exposure during this period , they note. "However, few investigators have studied this relation."

To investigate further, Li's group used data from the Women's Contraceptive and Reproductive Experiences Study (1994 to 1998), including 4,013 women with breast cancer and 4,069 women without breast cancer (the controls).

Among the white women, those who were premenopausal, had given birth and who also had an interval of at least 16 years from when they began to menstruate and their first birth had a 1.5-fold increased risk of breast cancer compared to those who had fewer than 5 years between these ages. This association was not observed among premenopausal African-American women.

The increased risks were mainly confined to women with hormone-receptor-positive tumors and tumors located in the breast lobules.

The results associating age at first full-term birth and breast cancer risk are generally consistent with previous results, but the findings regarding age at first menstruation are not, Li's team adds.

"Epidemiologic studies of both premenopausal and postmenopausal women have consistently found that breast cancer risk is reduced 5 to 20 percent for each year (menstruation) is delayed," they explain. However, in this study an older age at first (menstruation) was not associated with a reduced risk of breast cancer."

SOURCE: American Journal of Epidemiology, January 2008.