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Your app says your period is three days late. Before you read anything into that, it is worth knowing that the two health services most likely to be quoted at you cannot agree on what a regular cycle even is.
Two health services, two different answers
The NHS page on irregular periods (last reviewed 26 June 2026) says:
The average gap between periods starting (the menstrual cycle) is around 28 days.
and defines the boundary like this:
A woman's periods are irregular if the gap between them is less than 21 days or more than 35 days.
The US Office on Women's Health (updated 13 January 2025) draws it somewhere else:
Your periods are still "regular" if they usually come every 24 to 38 days.
Put those side by side:
| Counted as regular | |
|---|---|
| NHS (UK) | 21 to 35 days |
| Office on Women's Health (US) | 24 to 38 days |
A 36-day cycle is irregular in the UK and perfectly regular in the US. A 22-day cycle is the reverse. Neither body is wrong; they are drawing a line through a continuous range, and reasonable experts draw it in slightly different places.
So what does "late" actually mean?
It means your app made a prediction and the prediction did not come true.
That is worth saying plainly, because the word carries a weight it has not earned. "Late" sounds like a verdict about your body. It is a statement about a forecast — one your app generated from your own previous cycles, using a model that assumes those cycles describe a rhythm.
If your cycles vary, the app will produce that word often, and it will keep producing it no matter how healthy you are. Someone whose cycles honestly run between 26 and 38 days will be told they are "late" in roughly half of them, forever.
Why calendar prediction struggles
There is no mystery in it. Most period forecasts work from an average of your recent cycle lengths, and an average only describes a set of numbers well when those numbers are clustered.
Take two people, each with three recorded cycles:
| Cycles | Average | What the average tells you | |
|---|---|---|---|
| A | 28, 29, 28 | 28.3 | Nearly everything |
| B | 26, 38, 31 | 31.7 | Almost nothing |
Person B's average is 31.7, a length none of her cycles actually were. Predict the next period from it and the forecast is wrong before it is made — and then the error propagates, because that wrong prediction becomes part of the history the next one is built from.
This is also why ovulation and fertile-window estimates are shakier than period estimates in the same app. A period prediction is extrapolating from events you logged. A fertile window derived only from past dates is extrapolating from an extrapolation — the app did not observe your ovulation, it inferred it from a cycle length it also guessed.
That is a limit of calendar maths, not a flaw in one app. An app can only do better by observing something other than dates, which is the next section.
What to log instead of trusting the forecast
The Office on Women's Health suggests marking the day your period starts on a calendar, and recording:
How many days your period lasted: Was your period shorter or longer than the month before?
along with PMS symptoms, when bleeding starts, how heavy it is, and — for heaviness — "How many pads or tampons did you use?"
In practice, four things carry most of the value:
Start dates, consistently. The single most useful number you own. Even if you log nothing else that month, log the first day.
How long bleeding lasts, and how heavy. Pads or tampons used per day is a crude measure that doctors can actually work with, and it is far more concrete than "heavy".
The symptoms that actually bother you. Pain that stops you doing things, bleeding between periods, anything that changed. Not a score for everything every day — that is how people quit by week three.
If you want ovulation information, observe it rather than inferring it. Basal body temperature taken at the same time each morning, cervical mucus changes, or LH tests measure something happening in your body now. Calendar predictions do not. For irregular cycles this is the difference between a guess and an observation.
When it is worth seeing a doctor
Both sources are specific, and these are quoted rather than paraphrased because specifics matter here.
The NHS advises speaking to a GP "if your periods are irregular or your normal pattern of periods changes", and lists among its non-urgent advice:
your periods last longer than 7 days
you have bleeding between periods
It also flags irregular periods alongside other symptoms — weight gain, tiredness, facial hair growth, dry or oily skin — and irregular periods with difficulty getting pregnant.
The Office on Women's Health says:
Talk to your doctor or nurse if you have menstrual cycles that are longer than 38 days or shorter than 24 days.
It adds that skipping periods for three months when you are not pregnant or breastfeeding warrants a call, that "Bleeding that often lasts longer than eight days" is a sign of heavy bleeding, and:
Severe pain or bleeding that causes you to miss regular activities is not normal and can be treated.
That last line is the one worth re-reading. A lot of people put up with pain for years on the assumption that it is simply what periods are.
What to bring to the appointment
A short, specific record beats a long vague one. Three things:
- The start date of your last period, and the start dates before it if you have them.
- Your cycle lengths over the last few months, and how much they vary — the variation is often more informative than the average.
- A day-by-day log of the symptom that prompted the visit, covering two or three cycles if you can.
If your app can export a summary, take that rather than the phone. A single page you can hand over is easier for both of you than scrolling through screens in a ten-minute appointment.
A note on the app I build
Disclosed interest: Rosea is mine.
Two things in it come directly from the problems above. Its predictions adapt to your own cycle lengths rather than assuming a textbook 28 days, and it shows your cycle-length and regularity trends — so you can see how variable you actually are, which is the thing the word "late" obscures. For observing ovulation rather than inferring it, there is basal body temperature charting with automatic coverline confirmation, plus cervical mucus and LH test logging.
And for the appointment, it exports a clean PDF summary you can hand to a doctor. Everything stays on the phone — Rosea has no account and no server — which is a separate topic I have written about elsewhere.
It still cannot tell you whether your cycle is something to investigate. Nothing on your phone can. The two sections above are the ones that matter for that.
Frequently asked questions
What is a normal menstrual cycle length?
It depends which health service you ask. The NHS says the average gap is around 28 days and calls periods irregular if the gap is less than 21 or more than 35 days. The US Office on Women's Health says periods are still regular if they usually come every 24 to 38 days. Both are current official guidance.
Why does my period app say I am late when my period is not late?
Because the app is comparing today against a prediction it made from your previous cycles, and that prediction assumes a rhythm your body may not keep. "Late" is the app describing its own forecast as wrong, not a statement about your health.
Why are period predictions less accurate with irregular cycles?
Most forecasts work from an average of your recent cycle lengths. An average describes a set of numbers well only when they are clustered. If your cycles run 26, 38 and 31 days, the average is a number none of them were, and every prediction built on it inherits that error.
When should I see a doctor about irregular periods?
The NHS advises speaking to a GP if your periods are irregular or your normal pattern changes, if they last longer than 7 days, or if you bleed between periods. The US Office on Women's Health says to talk to a doctor or nurse about cycles longer than 38 days or shorter than 24, and notes that bleeding often lasting longer than eight days is a sign of heavy bleeding.
What should I track if my cycles are irregular?
Start dates and how long bleeding lasts, how heavy it is, and the symptoms that actually bother you. The Office on Women's Health suggests recording when bleeding starts, how heavy it is, how long it lasted, and how many pads or tampons you used. Several cycles of that is more useful to a doctor than any prediction.