Menstrual History

Women who started menstruating (having periods) younger than age 12 have a higher risk of breast cancer later in life. The same is true for women who go through menopause when they're older than 55. Over the past 15 years, girls have been starting puberty at younger ages. Breast development has started even earlier than menstrual periods. This unexpected shift has been attributed to the obesity epidemic and broad exposure to hormone disruptors, since a rise in hormones triggers the onset of breast development and puberty. The age when women go through menopause, however, has stayed about the same.
The earlier your breasts form, the sooner they're ready to interact with hormones inside and outside your body, as well as with chemicals in products that are hormone disruptors. This longer interaction with hormones and hormone disruptors can increase risk.
Also, when girls start menstruating at a younger age, the time between breast development and a first full-term pregnancy is usually longer than when menstruation happens later. During this time, breast tissue tends to be immature, overactive, and particularly sensitive to hormonal influences.
The longer a woman menstruates, the higher her lifetime exposure to the hormones estrogen and progesterone. All of these factors are associated with a higher risk of breast cancer later in life.