Your Cycle Is a Vital Sign

Since November 2006, the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics have told clinicians to record the menstrual cycle as a fifth vital sign, alongside temperature, pulse, breathing rate and blood pressure.

Four numbers get taken almost every time anyone sees a clinician: temperature, pulse, breathing rate, blood pressure. Nobody thinks of them as tests. They are the baseline, and a reading outside the expected range is a prompt to look further. The menstrual cycle now sits alongside them, and most people have never been asked for it.

How a period became a vital sign

ACOG and the AAP published Committee Opinion 349 in November 2006, revised it as Committee Opinion 651 in 2015, and have reaffirmed it since, most recently in 2025.

In November 2006 the American College of Obstetricians and Gynecologists published Committee Opinion 349, written by its Committee on Adolescent Health Care. The American Academy of Pediatrics ran the same text in Pediatrics that month. The title said it plainly: using the menstrual cycle as a vital sign.

It was revised in December 2015 as Committee Opinion 651, again endorsed by the AAP, and reaffirmed as current in 2025. Nearly twenty years on, it has not been softened.

The argument is short. Just as an abnormal blood pressure, heart rate or respiratory rate may be key to diagnosing a potentially serious health condition, ACOG argues that identification of abnormal menstrual patterns in adolescence may improve early identification of potential health concerns for adulthood. The instruction that follows is to clinicians: once someone starts menstruating, ask at every preventive visit for the first day of the last period and the pattern of periods, and record it with the other vitals.

What the guidance says the range actually is

The guidance sets a typical adolescent cycle at 21 to 45 days, a first period around age 12.43, and bleeding of seven days or less.

Vital signs only work against an expected range, and the guidance sets one out for adolescents. Median age at first period, 12.43 years, broadly stable between 12 and 13 across well-nourished populations. By 15, 98 per cent of girls have started. Mean cycle interval in the first gynaecological year, 32.2 days. Typical interval, 21 to 45 days. Flow of seven days or less, using three to six pads or tampons a day.

Around 90 per cent of adolescent cycles land in that 21 to 45 day window. By the third year after the first period, 60 to 80 per cent of cycles run 21 to 34 days, which is the adult pattern. For the mechanics behind those numbers, see our guide to your cycle's four phases.

Those numbers are not a target. They are a reference range, in the same sense that 120 over 80 is one.

The patterns clinicians treat as signals, and of what

Certain menstrual patterns, set out in the guidance's Box 3, are a reason to ask more questions, not a reason to panic.

ACOG's Box 3, which lists menstrual abnormalities that may require evaluation, sets out the patterns that warrant a proper look. Periods that have not started within three years of breast development, or by 15. Cycles shorter than 21 days or longer than 45. Any single gap of 90 days, the 95th percentile, so once is enough to warrant asking. Bleeding longer than seven days. Bleeding heavy enough to need a pad or tampon change more than once every one to two hours.

Each of those points somewhere. Long gaps with excess hair growth point towards hyperandrogenic anovulation, the family that includes the condition renamed in 2026 from polycystic ovary syndrome (PCOS) to polyendocrine metabolic ovarian syndrome (PMOS). Heavy bleeding, especially with easy bruising or a family history, points towards a clotting disorder such as von Willebrand disease. Periods that stop under heavy training or restricted eating point towards hypothalamic dysfunction. Thyroid disease, raised prolactin, pituitary problems and primary ovarian insufficiency all show up in the same place, on the calendar, before anywhere more obvious.

That is the whole case for the framing. The cycle sits downstream of a lot of systems, which makes it a cheap early warning. Any of these patterns, from a suspected clotting disorder to signs of PCOS/PMOS to periods that have not started on schedule, is worth raising with a GP rather than waiting to see whether it resolves on its own.

Why adolescence carries the most weight

Adolescence is the only window when an early deviation is visible as a deviation, because that is when the cycle's own pattern is still being established.

The guidance is written about girls and adolescents specifically, and that is deliberate.

Adolescence is when the pattern is being established, so it is the only moment an early deviation is visible as a deviation rather than as someone's lifelong normal. It is also the age at which people are least equipped to judge what is typical, and least willing to raise it. The committee is direct: patients and their families frequently cannot tell what counts as normal, and a patient may not tell a parent or carer, though they may confide in another trusted adult.

The wrong conclusion does damage in both directions. A teenager whose first two years are chaotic may be treated for a problem that does not exist. A teenager who has not bled for four months may be told to wait, when the guidance treats three months as the threshold for evaluation.

What a vital sign is not

A vital sign is a prompt to investigate, not a diagnosis, and no app can interpret an unusual cycle any more than an unusual blood pressure reading interprets itself.

Worth saying clearly, because the phrase gets borrowed by people selling things. An unusual blood pressure reading does not tell you what is wrong, and neither does an unusual cycle. Nothing in the guidance suggests the person recording it can interpret it, and no app can either.

It is also a reading that hormonal contraception changes. The pill, the injection, the implant and the intrauterine system all alter or remove bleeding, often deliberately. A withdrawal bleed on a combined pill is not the same signal as a natural cycle, and reading it as one is a mistake. Predictions built on any of this are estimates, and never a form of contraception, a point our piece on why your period app gets the date wrong explores further.

Carrying it into adult life

The published guidance stops at adolescence, but the NHS applies the same logic to adults, treating a cycle under 21 or over 35 days as irregular and a reason to see a GP.

The published position stops at adolescence. The reasoning does not.

In adults, the NHS treats a gap of under 21 days or more than 35 days as irregular, and flags bleeding longer than seven days, bleeding between periods, and three missed periods with a negative pregnancy test as reasons to see a GP. Same principle, different range.

The guidance also anticipated the practical obstacle. It tells clinicians that asking a patient to chart her cycles is worthwhile when the history is vague, that the counting rule needs explaining because it is a common source of miscommunication, and that smartphone apps make it easier. Cycle length runs from the first day of one period to the first day of the next. Not the last day. Not the gap in between.

The short version

Clinical guidance has treated the menstrual cycle as a vital sign since 2006, reaffirmed as recently as 2025, with a reference range for adolescents and a list of patterns worth a GP visit.

Roughly 90 per cent of adolescent cycles run 21 to 45 days, and a single gap of 90 days is worth a GP appointment on its own. In adults, the NHS treats a gap of under 21 days or more than 35 days as irregular. A vital sign is a prompt, not a diagnosis, and hormonal contraception changes the reading. None of it survives memory, which is the whole argument for writing down day one.

Simply Track exists to make that record cheap to keep. Tracking and the calendar are free, along with every first-period guide. The daily Log, Insights and the doctor PDF report come with Premium. There is no account, no ads, and cycle data stays on the device unless the owner syncs it to their own private iCloud.

Sources

• ACOG and AAP Committee Opinion 651, Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign (December 2015, reaffirmed 2025 per the ACOG webpage): https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2015/12/menstruation-in-girls-and-adolescents-using-the-menstrual-cycle-as-a-vital-sign
• AAP and ACOG, Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign, Pediatrics, November 2006: https://publications.aap.org/pediatrics/article/118/5/2245/69874/Menstruation-in-Girls-and-Adolescents-Using-the
• NHS, Irregular periods: https://www.nhs.uk/symptoms/irregular-periods/
• NHS, Periods: https://www.nhs.uk/conditions/periods/
• Endocrine Society, Polyendocrine Metabolic Ovarian Syndrome, the new name for the condition previously known as PCOS (12 May 2026): https://www.endocrine.org/news-and-advocacy/news-room/2026/pcos-name-change