Why do doctors mean by "hormone fluctuations"?

When a woman enters perimenopause – which can be as early as her mid- to late-30s – her levels of estrogen, progesterone, and testosterone begin to fluctuate. Most women already have experience with hormonal changes, as levels dip and rise during monthly cycles as well, but in this phase, the peaks are higher, the troughs deeper, and the trend is generally downwards. This the body beginning to ramp down reproduction: fewer eggs are released, cycles may get longer or shorter, heavier or lighter, mood can seem more volatile, sleep can be interrupted. Women’s brains are regulated to a very significant extent by estrogen, so as that hormone decreases, pretty much everything is affected. This period can last for years, though for most women it’s about four years in duration. Generally, the body adapts and finds its new normal as hormones level out, post-menopause.