A normal cycle is 24 to 38 days, not 28, and may bleed for up to eight

The International Federation of Gynecology and Obstetrics defines a normal menstrual cycle as one occurring every 24 to 38 days, with bleeding lasting up to eight consecutive days, and with the gap between a person's shortest and longest cycle staying inside an age-dependent limit — up to nine days for those aged 18 to 25 and 42 to 45, and up to seven days for those aged 26 to 41 [s1]. The familiar 28-day figure is an average, not a definition, and even as an average it is slightly off: across 612,613 real ovulatory cycles the mean length was 29.3 days [s3].

Anything falling outside those parameters is classified as abnormal uterine bleeding — which is a description of a pattern, not a diagnosis of anything.

Four questions, not one

FIGO maintains two systems, first published together in 2011 and revised through an iterative process between 2012 and 2017 involving its Menstrual Disorders Committee and invited epidemiologists and gynaecologists, with face-to-face meetings in Rome, Vancouver and Singapore and anonymous voting by modified RAND Delphi technique where substantial change was considered [s2]. System 1 names the bleeding pattern; System 2, known by the acronym PALM-COEIN, classifies the possible causes [s1].

System 1 defines a bleeding pattern using four separate descriptors — frequency, duration, regularity and flow volume — plus the presence or absence of intermenstrual bleeding, and of unscheduled bleeding for those using progestin-based hormonal contraception [s1]. All the metrics are based on the previous six months, provided there has been no pregnancy or puerperium [s1].

Frequency is the number of days from the first day of bleeding in one cycle to the first day of the next. Normal is 24 to 38 days; fewer than 24 is frequent menstruation and more than 38 is infrequent [s1]. Absence of menstrual bleeding without an iatrogenic cause is amenorrhoea, and is also abnormal [s1].

Duration counts consecutive bleeding days including light bleeding often called spotting; up to eight days is normal, and longer is prolonged [s1].

Regularity is the variance in days between the shortest and longest cycles, and it is the parameter most people do not know is measured separately [s1]. Below 18 and after 45, the guidance is explicit that ovulatory disorders are frequent and norms are not well defined [s1].

The reason for splitting the pattern four ways is diagnostic. Regular cycles at a normal frequency suggest ovulation is happening; irregular onset suggests an ovulatory disorder; intermenstrual bleeding may point toward a focal lesion such as a polyp [s1].

What the tracked cycles show

Norms defined by committee can be checked against data. An analysis published in npj Digital Medicine examined 612,613 ovulatory cycles from 124,648 users of a fertility-tracking app, using recorded menstruation, basal body temperature and luteinising hormone tests [s3].

Mean cycle length was 29.3 days [s3]. The mean follicular phase — from the start of bleeding to ovulation — was 16.9 days, with a 95% confidence interval spanning 10 to 30 days; the mean luteal phase, from ovulation to the next period, was 12.4 days with a much tighter interval of 7 to 17 days [s3]. In other words, the second half of the cycle is comparatively fixed and the first half does most of the varying, which is why counting backwards from a period is a poor way to locate ovulation.

Cycle length shortened with age: mean cycle length fell by 0.18 days per year of age between 25 and 45 (95% CI 0.17 to 0.18, R² = 0.99), and the follicular phase by 0.19 days per year (95% CI 0.19 to 0.20) [s3]. Variability differed by body-mass index too — mean variation of cycle length per woman was 0.4 days, or 14%, higher in women with a BMI over 35 relative to those with a BMI of 18.5 to 25 [s3].

The dataset is not a random sample of the population: it comes from people motivated enough to track temperature and hormone tests daily, and it was restricted to ovulatory cycles [s3].

Why the definitions exist at all

Abnormal uterine bleeding may affect over half of reproductive-aged women and girls, and heavy menstrual bleeding in particular is the largest contributor to iron deficiency and related anaemia — yet the review describes both as remaining underrecognised and underreported [s1].

Part of that is definitional drift. FIGO distinguishes chronic abnormal uterine bleeding, defined as "bleeding from the uterine corpus that is abnormal in volume, regularity, and/or timing, and has been present for the majority of the past 6 months", from acute abnormal uterine bleeding, "an episode of heavy bleeding that, in the opinion of the clinician, is of sufficient quantity to require immediate intervention to prevent further blood loss" [s1].

Part of it is social. The review notes that a structured history of the kind System 1 requires gives clinicians insight into problems patients have normalised on the basis of what family, friends or previous practitioners told them — something it identifies as especially true for heavy menstrual bleeding [s1].

One specific cause is systematically missed. For suspected coagulopathy, FIGO recommends a screening tool that is 90% sensitive for detecting a coagulation disorder, combining heavy menstrual bleeding present since menarche with any of: previous postpartum haemorrhage, surgical or dental bleeding, bruising at least one to two times a month, nosebleeds one to two times a month, frequent gum bleeding, or a family history of bleeding symptoms [s1]. A positive screen warrants further investigation with tests including von Willebrand factor and platelet function testing [s1].

The limits of a definition

These parameters describe patterns; they do not identify causes, and the FIGO cause classification exists precisely because the same pattern can arise from polyps, adenomyosis, fibroids, malignancy, coagulopathy, ovulatory dysfunction, primary endometrial disorders or medication [s1]. A cycle inside the normal ranges can still accompany a serious condition, and a cycle outside them is frequently benign.

What the definitions do is give a shared vocabulary for a conversation that often does not happen. Anyone whose bleeding pattern falls outside these ranges, or has changed, has a reason to describe it to a clinician using these four parameters — not a basis for concluding anything on their own.