Irregular Period Tracker: Predicting a Cycle That Doesn’t Follow the Rules

Irregular is a measured thing, not a feeling. The current international definition (FIGO 2018) describes a normal cycle on three counts, all of them judged over the previous six months. Frequency: a period starting every 24 to 38 days, counted from the first day of one to the first day of the next. Regularity: the spread between your shortest and longest cycle in that window — not the change from one cycle to the next — stays inside a limit that moves with age, 9 days or fewer from 18 to 25, 7 or fewer from 26 to 41, and 9 or fewer from 42 to 45. FIGO sets no norms below 18 or after 45, because irregular ovulation is expected at both ends. Duration: bleeding lasts up to 8 days.

Those are not the only numbers in use, and it is worth knowing that before you measure yourself against them. ACOG's patient guidance still describes a normal cycle as 21 to 35 days with a period of up to 7 days, while agreeing that cycle length varying by more than 7 to 9 days is irregular. The NHS says a period usually lasts about 5 days and can run anywhere from 2 to 7. A cycle can therefore be normal by one yardstick and not by another, which is a good reason to weigh a persistent change from your own normal more heavily than any threshold.

Fall outside the FIGO definition persistently and the cycle is, by definition, irregular: cycles shorter than 24 or longer than 38 days, a shortest-to-longest spread of 10 days or more, bleeding longer than 8 days, or no period for 90 days or more.

Two groups are irregular for expected reasons: the first few years after periods start, when ovulation is not yet reliable and cycles of 21 to 45 days are common (ACOG), and the perimenopausal years. Irregular does not automatically mean unhealthy. It does mean a calendar-based prediction will be wrong more often, and that a persistent change from your own normal is worth a conversation with a doctor.

Because they predict a day when the honest answer is a range. Almost every app adds the mean of your last few cycle lengths to your last period start and shows one confident date. For cycles running 27, 28, 29 days that works. For cycles running 26, 41, 30, 35, the mean is 33 and the date is often a week out, and the app looks broken when what is broken is the assumption.

The variability lives mostly in the follicular phase, from period to ovulation, which stress, illness, travel and poor sleep can lengthen by days or weeks; the luteal phase is comparatively stable. A calendar model cannot know that ovulation slipped this month.

The better approach is to report the spread as well as the centre: a likely window that widens when your history is more scattered, and a plain “later than usual” instead of a red overdue badge. PinkyBloom does this on iPhone, in its perimenopause mode, where cycles are expected to vary: the next period is shown as a window (“likely Jun 18–24”, then “likely any day now”), and a long cycle is treated as expected rather than as an anomaly. In cycle mode the app learns your average from the cycles you log and shows a single estimated day; read it as an estimate if your cycles are not regular.

Log fewer things, but log them every cycle. The record a clinician wants has four columns: the first day of bleeding, the last day, how heavy each day was (light, medium, heavy, or pads changed), and anything unusual, such as spotting between periods, clots, or pain out of the ordinary. Add a one-word note for the obvious disruptors when they happen: illness, travel, a bad month at work, a new medication, a big weight change. Those notes are what turn a 41-day cycle from a mystery into “the month I had flu”.

Give it three to six cycles. One irregular cycle is noise; three or more show whether you have a wide-but-stable pattern (say 30 to 42 days, every time), a trend (cycles getting steadily shorter or longer), or true chaos, and each of those points somewhere different clinically.

Watch the spread, not the average. Write down your shortest and longest cycle of the last year; that pair of numbers, plus bleeding length and any 90-day gap, is the summary your doctor will ask for.

And be explicit with yourself about what the record cannot do: with irregular cycles, an app’s fertile-window estimate is a guess about a moving target. Do not rely on it, or on any calendar method, for contraception.

Poorly. A calculator works backwards from an assumed next period — usually assuming a 14-day luteal phase, although Bull et al. measured a mean of 12.4 ± 2.4 days across 612,613 cycles, with 18% under 11 days — so its ovulation estimate inherits every error in the period prediction and adds its own. Across the 24-to-38-day normal range the ovulation day already spans two weeks; with genuinely irregular cycles the calculation is closer to a guess, and in anovulatory cycles, common with PCOS and in perimenopause, there is no ovulation to find.

If you need to know when you ovulate, use the body’s own signals rather than the calendar. Urine LH tests catch the hormone surge: from a positive stick, ovulation followed in about 20 hours in the study that measured it directly against ultrasound (20 ± 3 hours, 95% CI 14 to 26, in 26 women). The larger 24-to-36-hour figure you may have read is counted from the start of the surge in the blood, which a home test only registers some hours later — the two are not in conflict, they are measured from different moments. With long cycles you may need to test over more days, and PCOS can produce misleading LH readings. Basal body temperature, taken on waking, rises slightly after ovulation — ACOG puts the step at 0.5 to 1 °F, roughly 0.3 to 0.6 °C, while Mayo puts it lower, at less than half a degree Fahrenheit — and confirms in hindsight that it happened. Cervical mucus turning clear and stretchy signals the fertile days as they arrive. Used together and logged next to your periods, these give a far better picture than any date arithmetic, and over several cycles they also show whether you are ovulating at all, which is useful information for a doctor.

PinkyBloom shows a fertile window as an estimate from your dates, labelled as such. It is not a test, and it is not a method of contraception.

Polycystic ovary syndrome is the most common endocrine disorder in women of reproductive age — the 2023 international guideline puts its prevalence at 10 to 13% — and its cycle signature is specific. From about three years after your first period until perimenopause, the guideline counts a cycle as irregular when it runs shorter than 21 days or longer than 35, or when there are fewer than 8 cycles in a year. Either one qualifies; you do not need both. A single cycle longer than 90 days counts on its own from a year after your first period. The thresholds are looser in the years right after periods begin — 21 to 45 days between one and three years post-menarche — and the first year after menarche is explicitly normal, so the adult rule should never be applied to a young teenager.

Diagnosis uses the Rotterdam criteria, two of three: irregular or absent ovulation, signs of high androgens (on the skin or in blood tests), and polycystic ovaries on ultrasound or, in adults, a raised AMH — the 2023 guideline allows AMH in place of the scan, but not both, and not in adolescents. Where irregular cycles and high androgens are both present, neither the scan nor AMH is needed. Other causes such as thyroid disease and high prolactin are excluded first.

For the appointment, track what maps onto those criteria. Cycle dates and gaps over at least three to six months, since the cycle history is itself the first criterion. Skin and hair: acne, hair growth on the face or body, thinning at the crown. Weight, with the caveat that PCOS affects lean women too. Energy, sleep and mood, since they are part of the burden and can shift with treatment. Log the results of any home ovulation tests, remembering they can mislead in PCOS.

PinkyBloom’s symptom picker offers 24 items in cycle mode, filtered from a catalogue of 65; acne, mood and energy are in that list, hair changes are not, so write those down yourself. On iPhone the doctor-prep flow gathers what you logged, with your cycle dates, into a PDF report. Our PCOS guide, linked below, covers diagnosis and treatment in detail.

If you are in your 40s, or your late 30s, and cycles that used to be regular have started to wander, it may be. The clinical staging system for reproductive ageing, STRAW+10, defines the start of the early menopausal transition by a specific pattern: a persistent difference of seven days or more in the length of consecutive cycles, where persistent means it recurs within ten cycles of the first time. Note that this is a different measurement from FIGO’s, even though both land on seven days: STRAW compares one cycle with the next, FIGO compares your shortest with your longest. That is a tracker’s definition either way; you cannot see it without dates. The late transition begins when a gap of 60 days or more appears. Menopause itself is dated to the final period — you can only identify it in hindsight, once twelve months have passed without another one, by which time you have been postmenopausal for a year.

Other early signs often arrive alongside the cycle change: heavier or lighter flow than before, sleep breaking in the second half of the night, new premenstrual mood intensity, hot flushes or night sweats. Whether a hormone blood test helps depends on your age. From 45 onwards NICE says to identify perimenopause and menopause from symptoms and cycle history without laboratory tests at all, because levels swing from week to week. Between 40 and 45 with symptoms and a change in your cycle, and under 40 where menopause is suspected, NICE says the opposite: consider an FSH test there. In every band the cycle record and your symptoms are what the conversation starts from.

PinkyBloom has a perimenopause mode built for this: the symptom list changes to the 40 that matter at this stage, and on iPhone period prediction becomes a window instead of a date and a symptom-burden index tracks the overall load over time. Our perimenopause page and the answers linked below cover what to expect and when to see a doctor.

Irregular periods are worth a visit when they are new, persistent, or come with any of the signs below.

Book an appointment if cycles are persistently shorter than 24 days or longer than 38 days; you have had no period for 90 days and are not pregnant, breastfeeding or on a contraceptive that stops periods; bleeding lasts more than 7 days — the threshold ACOG and the NHS both use, a day tighter than FIGO's 8, so treat 8 as already worth raising; you bleed between periods or after sex; periods have stopped being regular after years of being regular; or you are trying to conceive and cycles are irregular.

Seek care the same day if you are soaking through a pad or tampon every hour for several hours in a row, passing clots larger than about 2.5 cm, feeling dizzy, faint or short of breath with heavy bleeding, have severe pelvic pain, or have heavy bleeding with a positive pregnancy test.

Any bleeding after menopause, meaning a year or more after your final period, needs assessment.

Bring your record: first and last days of each period for as long as you have them, your shortest and longest cycle, bleeding length, and the notes on what was going on. A doctor can act on six months of dates; “they’re all over the place” is where the conversation starts, not where it can end.