How long a cycle usually lasts, and what an irregular cycle does not say
Evidence last assessed 2026-10-09.
In short
- What the research shows
- FIGO's reference runs from 24 to 38 days, varying by up to 7 to 9 days across a year.
- What it doesn't show
- A pattern outside that reference does not say the cause; there are many possible.
- When it's worth raising with a professional
- It is worth raising with a clinician if your cycles stay outside 24 to 38 days or vary by more than 7 to 9 days across a year, if a bleed lasts longer than 8 days, if more than 90 days pass without bleeding while you are not on hormonal contraception, if you bleed between periods or after sex, or if the bleeding is heavy enough to change your day.
- Cycles
See studies and method
What the evidence says
An irregular cycle is a pattern, not a diagnosis. The international reference (FIGO, 2018) runs from 24 to 38 days, with up to 7 to 9 days between the shortest and the longest cycle in a year; outside that there are many possible causes, and your pattern alone points to none of them.
FIGO's System 1 (2018 revision) sets the reference for the reproductive years: 24 to 38 days from the first day of one bleed to the first day of the next; a cycle is regular when, across a year, the shortest and the longest are 7 to 9 days apart or less; and a bleed lasts up to 8 days. A pattern outside those parameters is called abnormal uterine bleeding, and FIGO's System 2 (PALM-COEIN) lists its possible causes: structural (polyps, adenomyosis, fibroids, malignancy and hyperplasia) and non-structural (clotting problems, ovulation that is not happening regularly, endometrial causes, causes from treatments, and others). The 2023 international guideline on polycystic ovary syndrome defines irregular cycles for that one diagnosis only (from three years after the first period to perimenopause: shorter than 21 or longer than 35 days, or fewer than eight cycles a year; any cycle longer than 90 days from one year after the first period) and uses them as one criterion among several, never as a diagnosis by themselves.
What this does not show
It is a population reference for the reproductive years, without hormonal contraception: it does not describe the first years after the first period or perimenopause, and on hormonal contraception the bleeds are withdrawal bleeds, not cycles. A pattern outside the reference is a reason to ask, not a finding: it names no cause, and Neta never guesses one. Your range, from your own cycles, comes first; the reference only gives it context.
Sources, as Neta holds them
Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet. 2018;143(3):393-408. doi:10.1002/ijgo.12666
Guideline synthesis
Teede HJ, Tay CT, Laven JJE, Dokras A, Moran LJ, Piltonen TT, et al. Recommendations from the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome. J Clin Endocrinol Metab. 2023;108(10):2447-69. doi:10.1210/clinem/dgad463
Guideline synthesis