Your waist is expanding and nothing has changed — same meals, same walks, same everything. Sound familiar? In perimenopause, it's not willpower. It's cortisol. As estrogen declines, it loses its buffering effect on your stress hormone — and cortisol responds by storing fat directly in your midsection. Here's the full hormonal story behind cortisol belly, why standard advice often makes it worse, and what the science actually says works.