What perimenopause actually is
Perimenopause is not the end of periods. It is the years before that point, when the ovaries begin to respond less predictably to the signals sent from the brain. Oestrogen does not fall in a straight line during this stage; it swings, sometimes reaching levels higher than in earlier reproductive life before dropping again. Progesterone tends to fall earlier and more steadily as ovulation becomes less regular. Those two facts explain almost everything women describe during this phase: cycles that shorten and then lengthen, bleeding that is heavier one month and barely present the next, and symptoms that seem to appear and disappear without any pattern.
For most women, perimenopause begins somewhere in the mid-forties and lasts around four to eight years, although it can be considerably shorter or longer. Menopause itself is a single retrospective point, confirmed after twelve consecutive months without a period when nothing else explains it. Everything before that point is perimenopause, and everything after it is postmenopause. Understanding where a woman sits on that timeline changes what her symptoms are likely to do next, which investigations are worth doing, and which treatments make sense now rather than later.