What it means
The cycle has four phases, and they are driven by hormones talking to each other.
Menstrual phase. Oestrogen and progesterone fall, and the uterine lining is shed and leaves as bleeding. Usually a few days, commonly 2-8.
Follicular phase. A follicle in one ovary develops and produces oestrogen. Energy and mood often rise in this phase. It varies the most between cycles.
Ovulation. A mature egg is released. Usually somewhere around the middle of the cycle, but the exact day varies and can be earlier or later than expected.
Luteal phase. Progesterone rises and prepares the lining. If no pregnancy, levels fall and the cycle starts again. PMS symptoms, if they occur for you, tend to be here.
What can vary
This is the part most people find least reassuring, so here is the actual detail:
- 21 to 35 days is considered regular. 28 is an average, not a requirement.
- Cycle length can change from cycle to cycle in the same person. A 24-day cycle followed by a 34-day one can be ordinary.
- Cycle length often gets more regular in the years after the first period, but some people never have a perfectly predictable one.
- Bleeding length varies from 2 to about 8 days.
- Perimenopause, usually in the forties and fifties, brings more variability. That is a normal life stage, not a problem in itself.
Possible explanations
If cycles are irregular outside those ranges, a clinician considers: PCOS; thyroid problems; high prolactin; very low body fat or very high energy expenditure; significant weight change; intense exercise; or, much more rarely in a way that causes irregularity, a structural or other gynaecological issue. A clinician cannot tell which from a description alone.
Warning signs
- Bleeding that soaks a pad or tampon every hour for several hours, with clots, particularly alongside dizziness, weakness, breathlessness or a racing heartbeat. This can cause anaemia and needs assessment.
- Bleeding between periods, or after sex, that is new and persistent.
- Periods that stop completely for three months or more while not pregnant — this needs checking for bone health, not just fertility.
- Severe pain that does not respond to usual pain relief and stops you doing normal activities.
- Any bleeding in someone whose periods have stopped for a year or more.
What to do next
Track what is actually happening — dates, how many days of bleeding, flow, and any pain. Apps help, and a paper diary works just as well.
If something fits the warning signs above, book an appointment rather than trying to manage it with products. That is the single most useful change many people make.
If cycles are irregular but you are otherwise well, it is still worth a routine appointment rather than an emergency one.
Self-care
- Products, not willpower, are the treatment for heavy bleeding. See period products.
- Heat genuinely helps cramps — a hot water bottle is not a folk remedy, it is a reasonable intervention.
- Movement tends to ease cramps for many people. Stretching and walking both help.
- Note that NSAIDs such as ibuprofen help menstrual pain if you can take them. Paracetamol is an option if you cannot.
- Nothing makes a cycle more regular except treating whatever is causing irregularity.
When to seek help
Routine: cycle questions, and irregular cycles that are not on the warning list.
Prompt: new pain, new spotting between periods, periods stopping for three months, or cycles outside 21-35 days that persist.
Emergency: soaking through protection hourly, bleeding with fainting or chest pain, or severe unremitting pelvic pain.
How to talk about it
“My cycles are about 35 days long. Is that worth checking?”
“I’m bleeding between periods and I don’t know why.”
“My period pain is stopping me going to school and pain relief isn’t working.”
Vague descriptions get vague answers. Day counts, cycle lengths and how many pads you changed get specific ones.
Related articles
Sources
Public sources are linked so you can read the detail rather than take this site’s word for it. None of this has yet been reviewed by a qualified clinician — see how we handle medical review.
References
Where this comes from
Linked so you can check it against a recognised public health source rather than take this site's word for it.
- The menstrual cycle — RCOG Royal College of Obstetricians and Gynaecologists (opens in a new tab)
- Menstruation and the Menstrual Cycle — ACOG American College of Obstetricians and Gynecologists (opens in a new tab)
These are public sources from recognised health organisations, linked so you can read the detail rather than take this site's word for it. Reading them does not tell you whether any of it applies to you, and none of these articles has yet been reviewed by a qualified clinician. How we handle sourcing and review.