Most women can name the average age of menopause, 51 in the UK, but the more useful number is quieter and less advertised: the transition that leads there usually opens several years earlier, in the mid-forties, and its first signs rarely announce themselves as hormonal. A shorter cycle here, a 3am waking there, a temper on a shorter fuse in the week before a period. Because the opening acts are so easily filed under stress, work or simply life, many women spend two or three years in perimenopause before anyone, including their GP, says the word. Knowing when it starts and what starting looks like converts that confusion into something navigable, and it changes the conversations you have with clinicians from vague complaints into a recognisable pattern with a name.
The ages, honestly stated
Perimenopause most commonly begins between 45 and 47, running around four years on average to the final period, though anywhere from a few months to a decade is documented. Starting at 40 to 44 is earlier than average and still within normal variation; around one in twenty women begins the whole process, transition and all, in this window. Before 40, changed rules apply: symptoms plus disrupted cycles at that age raise the question of premature ovarian insufficiency, which affects roughly one in a hundred women and needs proper investigation and treatment, not least to protect long-term bone and heart health. At the other end of the distribution, some women menstruate regularly into their mid-fifties and transition late. The width of this range is the point: your mother's timing is one of the better predictors available, and smoking reliably pulls the whole schedule one to two years earlier, but no calendar entitles or condemns anyone to a particular year.
What is actually happening hormonally
Perimenopause is not a smooth decline in oestrogen; it is instability. As the ovaries' remaining follicles dwindle, ovulation becomes erratic, and oestrogen lurches between higher-than-normal peaks and deepening troughs while progesterone, produced only after ovulation, falls away more steadily. This turbulence explains the signature strangeness of the phase: symptoms that come and go without pattern, months of normality followed by a bad quarter, cycles that shorten before they lengthen, and the disorienting sense of being fine in March and unrecognisable to yourself by June. It also explains why a single hormone blood test is so unhelpful after 45: a snapshot of a lurching system can look entirely normal on the day of the draw. UK guidance accordingly tells clinicians to diagnose perimenopause in women over 45 on the pattern of symptoms and cycles alone, reserving blood tests mainly for women under 45, and especially under 40.
The early signs most people miss
Hot flushes are the celebrity symptom but they are usually a middle chapter, not the opening one. The earliest reliable sign is the cycle itself changing character: typically shortening first, from 28 days towards 24 or 25, becoming heavier or lighter, then later skipping and stretching. Around it cluster the quieter arrivals. Sleep fragments, classically with 3am or 4am waking that has no obvious cause. Premenstrual symptoms sharpen, and mood dips or anxiety appear in women who never previously struggled. Concentration and word-finding develop occasional gaps, the much-discussed brain fog. Joints ache without injury. Libido drifts. Migraines can change pattern. Any one of these alone proves nothing, which is exactly why the phase goes unrecognised; it is the accumulation across a woman in her forties, especially alongside cycle change, that makes the picture. A symptom diary kept for two or three cycles is frequently more diagnostic than any test, and it converts a GP appointment from ten minutes of trying to remember into a pattern a clinician can actually read.
When to seek advice, and what help looks like
The threshold for a conversation is impact, not severity theatre: if symptoms are affecting sleep, work, relationships or how you feel about yourself, that is sufficient. Seek advice promptly, whatever your age, if cycles have changed dramatically, if bleeding is very heavy, prolonged or occurs between periods or after sex, or if you are under 40 with menopausal-pattern symptoms; these need assessment in their own right. For confirmed perimenopause, the toolkit is genuinely good. HRT is effective for flushes, night sweats, sleep and mood symptoms and can be started during perimenopause, not only after periods stop; for most healthy women under 60 the benefits outweigh the risks under current UK guidance. Body-identical progesterone alongside oestrogen protects the womb; local vaginal oestrogen addresses dryness independently. Non-hormonal routes, from CBT to certain antidepressants, serve women who cannot or prefer not to take hormones. And the unglamorous levers, strength training, alcohol moderation, regular sleep timing, steady up the baseline everything else works against, protecting bone and muscle through the years when both need it most.
The honest summary: perimenopause usually opens in the mid-forties, announces itself through cycles and sleep before it ever produces a hot flush, cannot be ruled in or out by a single blood test after 45, and responds to treatment at any point along its course. The women who fare best tend to be the ones who recognised the phase early, tracked what was actually happening, and asked for help calibrated to impact rather than waiting for some imagined threshold of sufficient suffering. For the full symptom landscape, see our complete guide to perimenopause symptoms, and for the testing question specifically, our explainer on perimenopause tests.
