This is the single most-misunderstood medical question in women's health. Two decades of post-WHI re-analysis have meaningfully shifted what evidence actually says.
What the 2002 WHI actually showed
In the combined arm (conjugated equine estrogens + medroxyprogesterone acetate), after 5.2 years average follow-up, breast cancer was elevated per 8 extra cases per 10,000 woman-years. Headlines reported "26% increase" — true relatively but misleading absolutely. The average WHI participant was 63 years old, an average 12 years past final menstrual period — not the typical symptomatic perimenopausal woman.
The estrogen-only arm tells a different story
Women without a uterus (no progestin needed) on conjugated equine estrogens alone showed DECREASED breast cancer incidence in WHI, confirmed in 2020 long-term follow-up. This finding got minimal coverage but is foundational to understanding that estrogen alone behaves very differently from estrogen-plus-progestin.
Timing matters substantially
The "timing hypothesis" — supported by re-analyses, ELITE, and KEEPS trials — shows HRT initiated within 10 years of FMP or under age 60 has neutral-to-favorable cardiovascular outcomes and a modest breast cancer risk increase that is generally outweighed by symptom relief, bone protection, and other benefits in appropriate candidates.
The honest answer: combined HRT increases breast cancer risk modestly in some women, no risk increase in others, and decreased risk in some configurations. Individual eligibility requires a clinician familiar with current evidence — not 2002-era guidance.