Detailed definition
The WHI estrogen-plus-medroxyprogesterone arm (CEE 0.625 mg + MPA 2.5 mg daily, mean treatment ~5 years) showed an increase of approximately 8 invasive breast cancers per 10,000 woman-years over placebo — a small absolute risk increase. The estrogen-alone arm (women post-hysterectomy on CEE) actually showed a modest reduction in breast cancer incidence and mortality. The French E3N cohort study and several meta-analyses subsequently showed that the breast cancer signal in WHI was largely driven by the synthetic progestin medroxyprogesterone, with bioidentical micronized progesterone showing a much smaller or absent signal at 5 years (with some increase appearing at longer durations). Modern menopause practice generally communicates breast cancer risk as: small risk increase with combination HRT (lower with bioidentical progesterone than with synthetic progestins) appearing after several years of use, balanced against the substantial symptom and bone benefits and the absence of any breast cancer increase in shorter-duration estrogen-alone therapy.
Why it matters in menopause
Breast cancer risk is the single most-asked-about HRT safety question and the one most distorted by 2002 headlines. The honest version: small risk, modulated by progestogen choice and duration, balanced against substantial symptom and bone benefits — and substantially offset by the cardiovascular and bone benefits of HRT in women in the timing window. ClearedRx discusses this individualized math at intake.
Related terms
Sources
External references: Wikipedia.