Perimenopause is the stretch of years before your final period when the hormones governing your cycle stop behaving predictably. It commonly starts in your forties, sometimes the late thirties, and it can last four to ten years.
It is defined by instability rather than decline, which is exactly why it is so hard to recognise from the inside. Symptoms come and go. A blood test on one day can look entirely normal. Two women the same age can have completely different experiences.
Most of what people call "going through menopause" is actually perimenopause.