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Hormones & cycle 2 min read

Irregular cycle: the possible causes and when to worry

An irregular cycle can come from stress, a change in weight, a thyroid working too fast or too slow, PMOS (previously PCOS), coming off contraception, or intense training combined with an energy deficit. Irregularity lasting beyond three consecutive cycles, or a sudden break from your usual pattern, is worth a consultation.

Macro texture standing for the rhythm of the menstrual cycle and its variations

A cycle that shifts is one of the most common reasons women start paying attention to their hormones — and one of the least specific. Almost everything that affects the body’s energy balance can affect it, which is why the useful question is rarely "why is it irregular" but "what changed, and for how long".

What regulates a normal cycle

The menstrual cycle runs on a precise conversation between the brain and the ovaries, and that conversation is sensitive to how much energy the body has available. A regular cycle is a range rather than a number, and the range itself depends on who you ask: current FIGO criteria put it at 24 to 38 days, while many public health services still publish the older 21 to 35 day window. Either way there is no single universal reference cycle, and a small variation from one month to the next stays well inside normal.

The most frequent causes

Chronic stress can delay ovulation or suppress it altogether. Rapid weight loss or a significant calorie deficit sends a restriction signal that can interrupt cycles — the energy deficit matters more than the weight change itself. A thyroid running too fast or too slow is a classic. PMOS (previously PCOS) is associated with irregular cycles, usually alongside other signs such as acne and increased hair growth. And after stopping hormonal contraception, cycles can take a while to settle while ovulation re-establishes itself.

When to see someone

Beyond three consecutive irregular cycles, after a sudden break from your usual pattern, with very heavy bleeding, or with no period for more than three months outside of pregnancy — a consultation is the right step rather than waiting for it to settle on its own.

  • Record your cycle lengths for at least three months, to turn what you are feeling into something you can show.
  • Check whether the irregularity lines up with an event: stress, weight loss, coming off contraception, a jump in training volume.
  • Do not conclude PCOS on your own — several other causes produce the same pattern.
  • See a professional beyond three consecutive irregular cycles, or after any abrupt change.

Reviewed by the OVEA medical committee

Frequently asked questions

How long can a cycle stay irregular before it is a concern?

One or two irregular cycles are common and usually harmless; beyond three consecutive ones, it is worth being seen.

Can stress really move my period?

Yes — significant stress can delay or suppress ovulation through its effect on the hormonal axis in the brain.

Does an irregular cycle mean infertility?

No. It may simply reflect less frequent ovulation, which is not infertility, though it can mean a plan to conceive needs support.

Can exercise disturb the cycle?

A high training volume combined with insufficient energy intake can interrupt ovulation in some women.

What counts as a regular cycle?

Roughly 24 to 38 days under current FIGO criteria, or 21 to 35 in the older window many health services still publish — what matters more is your own usual pattern.

Sources & method

This article was written from reference medical guidelines and publications, then reviewed by the OVEA medical committee.

  1. Munro MG, Balen AH, Cho S, et al. The FIGO Ovulatory Disorders Classification System. Human Reproduction 2022;37(10):2446–2464. DOI: 10.1093/humrep/deac180
  2. NHS. Irregular periods.
  3. Gordon CM, Ackerman KE, Berga SL, et al. Functional Hypothalamic Amenorrhea: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2017;102(5):1413–1439. DOI: 10.1210/jc.2017-00131
  4. Assurance Maladie. Qu’est-ce qu’une aménorrhée et quand faut-il s’inquiéter de l’absence de règles ? Ameli.fr (2026).

Health note

This content is educational and does not replace a diagnosis, a treatment or a consultation with a health professional. If symptoms persist or worry you, see a doctor.

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