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: Estrogens are natural hormones important for sexual development and other body functions. Before menopause, they are produced mainly in the ovaries. After menopause, they are produced mainly in fat tissue.
Because of the different sources (and different levels) of estrogen in women before and after menopause, it's important to look at studies of estrogen and breast cancer risk by menopausal status.
Studies show higher blood levels of the estrogen called estradiol increase the risk of breast cancer in postmenopausal women.
The studies below excluded women who were taking 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 don't take (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 7 studies found 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 [13-14]
Pooled and meta-analyses
Walker et al. 
Manjer et al. 
* Relative risk for estrogen receptor-positive and progesterone receptor-positive (ER-positive/PR-positive) breast cancers was 2.8 (2.0-4.0). Relative risk for estrogen and progesterone receptor-negative (ER-negative/PR-negative) breast cancers was 1.1 (0.6-2.1).
† Relative risk for ER-positive cancers only. Relative risk for ER-negative breast cancers was 1.65 (0.91-2.98).
‡ Results are for estradiol blood levels measured in the follicular phase of the menstrual cycle. Results for estradiol levels measured in the luteal phase of the menstrual cycle were not statistically significant.
§ Results are for estradiol blood levels measured in the follicular phase of the menstrual cycle. Results for estradiol levels measured in the luteal phase were similar with a relative risk of 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-positive cancers only. Authors noted an increase in risk before adjustment for blood testosterone levels.
** Results are for estradiol blood levels measured in the luteal phase of the menstrual cycle.
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