Glossary · Research

MWS (Million Women Study)

Also called: Million Women Study.

Definition: The Million Women Study is a large UK observational cohort study of more than a million postmenopausal women that has reported on HRT and breast, ovarian, and other cancer risks. It contributed to the post-WHI concern about HRT and breast cancer but, as an observational study, has limitations including non-randomization and recall bias.

Detailed definition

The Million Women Study (MWS) was launched in 1996 in the UK and recruited women through the National Health Service Breast Screening Programme, ultimately enrolling over 1.3 million women. It collected data on HRT use and tracked subsequent cancer incidence. MWS reported associations between current HRT use and increased breast cancer incidence (particularly with combination estrogen-progestin therapy) and increased ovarian cancer incidence with long-term use. Because it is observational rather than randomized, MWS carries the usual limitations: women choosing HRT differ from non-users in many ways, recall and reporting biases affect HRT classification, and confounders may not be fully controlled. The MWS findings are generally directionally consistent with WHI for breast cancer (though with the caveats of observational design), and have been an important data source for understanding longer-duration HRT use.

Why it matters in menopause

MWS is part of the broader evidence base on long-duration HRT and cancer risk, alongside WHI and the French E3N cohort. For most clinical decisions, the WHI randomized data and the timing-hypothesis interpretation dominate the evidence picture; MWS contributes nuance about longer-duration use.

Sources

External references: Wikipedia.

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