Osteoporosis is usually filed under "things to worry about after menopause." But the bone loss that leads there doesn't wait for your final period. It has a specific, measurable start point — and it falls inside the perimenopausal years, while most women are still having periods and have no idea it's underway.

The window opens in late perimenopause

SWAN tracked bone mineral density in the same women across the transition and found something precise: for most of perimenopause, bone density holds relatively steady — and then, in late perimenopause, loss accelerates detectably. It doesn't wait for menopause to begin.

Zooming in tighter, the fastest loss clusters around the final menstrual period. Bone loss begins to speed up roughly one year before the final period and continues rapidly for about two years after, before slowing (but not stopping). This roughly three-year transmenopausal window is the fastest sustained bone loss of a woman's life.

How much bone is at stake

Over that transmenopausal window, SWAN measured average losses of roughly 7% at the spine and 5% at the hip. That is a meaningful bite taken out of the skeleton in just a few years — and because it happens quietly, without symptoms, it's easy to miss until a fracture or a scan reveals it later.

Who loses the most

The rate isn't the same for everyone. SWAN found that thinner, lower-body-weight women tend to lose more bone during the transition, independent of race and ethnicity — a useful flag, because smaller-framed women are sometimes wrongly assumed to be fine. Hormonal markers tracked too: higher estradiol and lower FSH were linked to slower spine bone loss.

Why the timing is the point

This is one more example of perimenopause being an active phase, not a waiting room. Knowing the loss starts in late perimenopause changes what "prevention" means: the habits that protect bone — adequate calcium and vitamin D, regular weight-bearing and resistance exercise, not smoking — matter now, in the years you can still bank and defend bone, not after the fastest losses are behind you. For women with extra risk factors, it's also a reason to ask a clinician whether a baseline bone-density scan makes sense.

This article is for general information and is not medical advice. Talk with a qualified clinician about bone health, screening, and what's right for you.