Causes of Breast Cancer: Hormonal Factors

Being exposed to estrogen over a long period of time may increase the risk of breast cancer.

What is Endogenous Estrogen?

Endogenous estrogen is a hormone made by the body. It helps the body develop and maintain female sex characteristics.

Estrogen levels are highest during the years a woman is menstruating.

A woman's exposure to estrogen is increased in the following ways:

  • Early menstruation: Beginning to have menstrual periods at age 11 or younger increases the number of years the breast tissue is exposed to estrogen.
  • Late menopause: The more years a woman menstruates, the longer her breast tissue is exposed to estrogen.
  • Late pregnancy or never being pregnant: Because estrogen levels are lower during pregnancy, breast tissue is exposed to more estrogen in women who become pregnant for the first time after age 35 or who never become pregnant.

Hormone Replacement/Hormone Therapy

Hormones that are made outside the body, in a laboratory, are called exogenous hormones.

Estrogen, progestin, or both may be given to replace the estrogen no longer produced by the ovaries in postmenopausal women or women who have had their ovaries removed.

This is called hormone replacement therapy (HRT) or hormone therapy (HT).

Studies show that when women stop taking estrogen combined with progesterone, the risk of getting breast cancer decreases.