This summary table contains detailed information about research studies. Summary tables offer an informative look at the science behind many breast cancer guidelines and recommendations. However, they should be viewed with some caution. In order to read and interpret research tables successfully, it is important to understand some key concepts. Learn how to read a research table.
Introduction: Introduction: Estrogens are natural hormones that are important in sexual development and other body functions. They are produced mainly in the ovaries in premenopausal women, and mainly in fat tissue in postmenopausal women.
Because of the different sources (and different levels) of estrogen in premenopausal and postmenopausal women, it is important to look at studies of estrogen and breast cancer risk by menopausal status.
The studies listed in the table below show that higher blood levels of the estrogen called estradiol increase the risk of breast cancer in postmenopausal women.
All of the studies below excluded women who were using menopausal hormone therapy (MHT) at the time of blood collection. Certain MHT might mask the full effect natural blood estrogen levels have on breast cancer risk. By looking only at women who are not taking (or who have never taken) MHT, researchers may see more clearly how blood estrogen levels affect breast cancer risk.
A pooled analysis of data from seven studies found that higher blood estrogen levels increased breast cancer risk in premenopausal women .
Learn more about blood estrogen levels and breast cancer risk.
Learn about the strengths and weaknesses of different types of studies. See how this risk factor compares with other risk factors for breast cancer. Study selection criteria: Prospective nested case-control studies with at least 100 breast cancer cases, pooled analyses and meta-analyses. Table note: Relative risks above 1 indicate increased risk. Relative risks below 1 indicate decreased risk.
Risks related to total estradiol levels are shown in this table.
Study Population(number of participants)
Risk of Breast Cancer in Women with Higher Estradiol LevelsCompared to Women with Lower Estradiol LevelsRelative Risk (95%CI)
Nested case-control studies
Nurses' Health Study 
EPIC cohort [3-4]
New York University Women's Health Study 
Nurses' Health Study II 
Falk et al. 
UK Collaborative Trial of Ovarian Cancer Screening 
Melbourne Collaborative Cohort Study 
Study of Osteoporosis Fractures Research Group [11-12]
ORDET cohort 
Pooled and meta-analyses
Walker et al. 
Manjer et al. 
* Relative risk for estrogen receptor-positive and progesterone receptor-positive (ER+/PR+) breast cancers was 2.8 (2.0-4.0). Relative risk for estrogen and progesterone receptor-negative (ER-/PR-) breast cancers was 1.1 (0.6-2.1).
† Relative risk for ER+ cancers only. Relative risk for ER- breast cancers was 1.65 (0.91-2.98).
‡ High versus low levels of follicular estradiol in women at the same point in their menstrual cycle. Results for luteal estradiol levels were not statistically significant.
§ High versus low levels of follicular estradiol in women at the same point in their menstrual cycle. Results for luteal estradiol levels were similar with a relative risk 1.2 (1.0-1.6).
|| Relative risk for women with higher levels of free estradiol compared to women with lower levels of free estradiol was 1.75 (1.06-2.89).
¶ Relative risk for ER+ cancers only. Authors noted an increase in risk before adjustment for blood testosterone levels.
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