Hormones and Breast Cancer Risk: What’s the Connection?

About 1 in 8 U.S. women will develop invasive breast cancer over the course of their lifetime.

FACT: Over 75% of women with breast cancer have no family history of the disease. 

Did you know? Breast cancer risk is associated with exposure to hormones (estrogen and progesterone) produced in a woman’s ovaries. The longer a woman is exposed to hormones produced by her body, the greater the risk of breast cancer.

Increased exposure to hormones = increased breast cancer risk

Reproductive factors that increase breast cancer risk include:

• Early menstruation (before age 12)

• Late menopause (after age 55)

• Never giving birth

Pregnancy and breast cancer risk

Both pregnancy and breastfeeding reduce breast cancer risk because they lower the number of menstrual cycles(hormone exposure) a woman will have in her lifetime. But different pregnancy circumstances can raise or lower breast cancer risk.

What raises breast cancer risk?

• First pregnancy at an older age 

• Recent childbirth (temporary increase that declines after about 10 years)

What reduces breast cancer risk?

• First pregnancy at an early age 

• Each additional pregnancy (the more you have, the lower your risk)

• Breastfeeding for at least a year

Birth control and breast cancer risk

• There is some evidence that hormonal contraceptives like the pill increase breast cancer risk, but overall, risk of breast cancer among birth control users is low. 

• Hormonal contraceptives are linked to a lower risk of ovarian, endometrial and colon cancers. 

Hormone replacement therapy (HRT) and breast cancer risk

• There are two kinds of HRT: combination estrogen and progesterone and estrogen-only.

• Recent research indicates that combination HRT increases breast cancer risk significantly, while estrogen-only HRT doesn’t increase risk unless it’s used for more than 10 years.

This resource was created with support from Daiichi Sankyo and Merck.