Women can lose 5–10% of bone mineral density before their last period — silently, with zero symptoms. Perimenopause is the fastest bone loss window of a woman's life, yet almost no one is talking about it. Here's the DEXA scan science, the T-score you need to know, the exercise hierarchy that actually rebuilds bone, and the calcium + vitamin D + K2 + magnesium stack the evidence supports — plus why the perimenopausal window to act is right now, not after your diagnosis.