Ask a woman when perimenopause started and she'll usually name a symptom — the summer she couldn't sleep, the year her temper went strange. Ask her about her cycle in the eighteen months before that, and about half the time something surfaces. Periods that arrived early. A month that was heavier than any she'd had since her twenties. A skipped one she put down to a stressful spring.
The cycle changes first. It's the most reliable early signal there is, and it's the one most likely to be dismissed — by you, and sometimes by the person you tell.
What actually changes, and in what order
Cycles get shorter. This is usually the opening move and it surprises people, who expect things to stretch out. In the late reproductive years the follicular phase — the stretch between your period starting and ovulation — often contracts. A lifelong 28-day cycle becomes 26, then 25. You don't notice, because you're not comparing.
Then cycles get erratic. Short one month, long the next. The formal marker for the start of perimenopause is a persistent seven-day-or-more difference between consecutive cycles. Not a single unusual month — a pattern that keeps recurring.
Then bleeding changes character. Heavier, or lighter, or the same volume compressed into two ferocious days. Clots appear where there weren't clots. Some women get flooding — genuinely disruptive bleeding that changes what they can plan.
Then the gaps. Sixty days, then a period. Ninety days, then a period. This is the late transition, and it usually means the end is within a few years.
Why it happens
The standard explanation — estrogen declining — is not quite right, and getting it right explains the mess.
What declines steadily is the number and quality of remaining follicles in your ovaries. In response, FSH rises, effectively shouting louder to get a follicle to develop. Sometimes it overshoots and you recruit follicles fast, producing a short cycle and a high estrogen surge. Sometimes nothing develops at all and you don't ovulate.
Progesterone is only produced after ovulation. So in an anovulatory cycle, you get estrogen without the progesterone that normally balances it and triggers a clean, organised shed. The lining builds up unopposed and then comes away disorganised — which is exactly what heavy, unpredictable bleeding is.
That's why the first phase of perimenopause often feels like too much rather than too little. Sore breasts, heavier bleeding, worse premenstrual weeks. It isn't a shortage yet. It's a loss of rhythm.
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What's normal and what isn't
Perimenopausal bleeding is expected to be irregular. But irregular has limits, and some patterns should be looked at rather than absorbed:
- Bleeding that soaks through protection hourly, or that stops you leaving the house - Periods lasting longer than seven days when they never used to - Bleeding between periods - Bleeding after sex - Any bleeding at all after twelve period-free months
None of these mean something is wrong. They mean something should be examined. Fibroids, polyps and endometrial changes are common in this age group, treatable, and sometimes the actual explanation for the exhaustion you've been attributing to hormones.
Heavy bleeding also depletes iron. Low ferritin produces fatigue, breathlessness and brain fog that are almost indistinguishable from what people call "menopause symptoms". It's a simple blood test and a genuinely worthwhile question to ask.
What to record
You don't need an elaborate system. Three things, for three or four months:
Start date of each period. From this you get cycle length, which is the entire diagnostic signal.
A rough note on flow. Light, normal, heavy, or "I had to change plans."
Anything unusual. Spotting, bleeding between periods, a period that came twice in one month.
Four months of that is more useful in an appointment than any hormone test you're likely to be offered. It converts "my periods have been all over the place" into a pattern someone can read — and it is the single piece of preparation most likely to change how the conversation goes.
Sources
- Harlow SD, Gass M, Hall JE, et al. Executive summary of the Stages of Reproductive Aging Workshop +10. Menopause. 2012;19(4):387–395. - National Institute for Health and Care Excellence. Menopause: identification and management (NG23). 2015, updated 2024. - National Institute for Health and Care Excellence. Heavy menstrual bleeding: assessment and management (NG88). - Santoro N. Perimenopause: from research to practice. Journal of Women's Health. 2016;25(4):332–339.