During the reproductive years (from menarche to menopause), a woman’s ovaries produce estradiol, the most potent estrogen, and progesterone. These two hormones have distinct patterns during menstrual cycles in which ovulation occurs. A woman’s ovaries also produce a small amount of testosterone. 

Natural menopause

Natural menopause is when menstrual cycle activity stops without surgical removal of both ovaries. The average age at natural menopause for women living In the United States is 51 years and the majority of women experience natural menopause between the ages of 44 and 57 years.

Natural menopause occurs gradually, during a transition period known as the perimenopause. During this time, women’s menstrual cycles become irregular as the time between menstrual bleeding episodes increases, and the overall amount of estradiol and progesterone produced by the woman’s ovaries gradually decreases.

The end of this transition period, which is called “menopause,” occurs when menstrual periods stop altogether. Although the woman experiences no more menstrual periods, her ovaries are still producing a small amount of estradiol, which stops in about a year. Her ovaries continue to produce a small amount of testosterone; this helps to maintain bone and reduce risk of osteoporosis.

Exposure to Endocrine-disrupting chemicals

A possible concern during the menopausal transition is the effects of exposure to “endocrine-disrupting chemicals,” chemicals that can mimic or interfere with the function of hormones in the body. 

Not all endocrine disruptors are man-made chemicals; they also may be naturally-occurring compounds. These endocrine- disrupting compounds can change hormonal activity by turning on, turning off or modifying the signals that hormones carry. These effects can be very important during the menopausal transition, a time when estrogen exposure is low. 

Because the majority of breast cancers are estrogen (and/or progesterone) responsive, increasing estrogen (or estrogen plus progesterone) levels during this period by exposure to hormone therapy or endocrine disrupting chemicals, may cause tiny, insignificant breast lesions to develop into clinically detectable tumors.