Irregular Periods: Causes, Tests, and What Actually Helps

Almost everyone who menstruates has an irregular stretch at some point: a cycle that runs long after a hard month, two periods close together, a gap that stretches past six weeks. The question that matters is not whether your periods are irregular right now, but which kind of irregular you are dealing with - the kind that settles on its own, or the kind that is your body reporting something worth checking.

This guide is the hub for everything on this site about irregular cycles. It defines "irregular" using the thresholds doctors actually use, groups the causes by what is going wrong, lists the symptoms that point to each cause, walks through the tests, and explains what helps. Where we have a deeper article on a specific cause, it is linked.

What "irregular" actually means

Most people use "irregular" to mean "not every 28 days". Doctors use narrower definitions. The most widely used come from FIGO, the International Federation of Gynecology and Obstetrics, whose 2018 classification describes a normal cycle in four dimensions:

Two things stand out. First, a "normal" cycle is not 28 days but a range: the NHS gives 21 to 40 days as the spread seen across healthy adults, Mayo Clinic gives 21 to 35, and FIGO's stricter definition 24 to 38. A 33-day cycle every month is regular. Second, regularity is about variation, not length. If your cycles run 30, 31, 29, 31, you are regular. If they run 24, 38, 27, 41, you are irregular even though each cycle falls inside the normal range.

The NHS uses a simpler rule of thumb for when irregularity is worth a GP visit: a difference of 20 days or more between your shortest and longest cycle.

For the basics, see what is a normal cycle length and how to tell if your cycle is irregular.

How to measure it yourself

You cannot judge regularity from one cycle, and memory is unreliable. Three steps:

1. Count from day 1 to day 1. Day 1 is the first day of real bleeding, not spotting. The cycle ends the day before the next period starts. How to calculate your cycle length walks through it.
2. Log at least three cycles, ideally six. Regularity is a pattern. The menstrual cycle calculator maps a single cycle; the period calculator projects your next dates from your average.
3. Write down the shortest and longest. The difference between them is your regularity number. Under 8 days is regular. 20 or more is the NHS threshold for getting checked.

When irregular is expected

Four life stages produce irregular cycles that usually need no treatment:

• The first years after your first period. ACOG's committee opinion on adolescent menstruation notes that cycles in the first one to two years after menarche are often anovulatory and commonly range from 21 to 45 days; by the third year most settle into an adult pattern. See your first period: signs and what to expect.
• Perimenopause. A persistent difference of 7 or more days between consecutive cycles is one of the first markers of the menopause transition, and gaps of 60 days or more mark the later stage. See how perimenopause is diagnosed.
• After a baby. Breastfeeding suppresses ovulation, and the first cycles back are often irregular. See postpartum periods.
• Coming off hormonal contraception. Most people ovulate within one to three months of stopping the pill, patch, ring, implant, or hormonal IUD. The injection can take up to a year. Irregular bleeding in that window is expected; no period at all after three months is worth a check. See tracking your cycle on birth control.

The causes, grouped by mechanism

Doctors sort abnormal bleeding into two buckets, structural and non-structural, using a FIGO scheme called PALM-COEIN. For irregular timing, the non-structural bucket, and specifically ovulation problems, is by far the most common. It helps to think in four groups.

1. Things that disrupt ovulation for a cycle or two

Ovulation is triggered by a signal chain running from the brain (hypothalamus and pituitary) to the ovaries. Anything that stresses the system can delay or skip it, and a late ovulation means a late period.

• Stress. Cortisol interferes with the hormones that trigger ovulation. A bad month can push a period back a week or more. See can stress delay your period.
• Hard training and under-fuelling. The problem is less the exercise than the energy deficit; cycles lengthen and then stop when intake does not cover training. See can exercise delay your period.
• Rapid weight change in either direction changes estrogen production and can stall ovulation.
• Sleep disruption, jet lag, and shift work shift the body clock that times the ovulation signal.
• Illness and fever can delay a single ovulation.

A single late period usually belongs here. Late period: 12 reasons beyond pregnancy covers the one-off case; this guide is for when it keeps happening.

2. Hormonal conditions that disrupt ovulation every cycle

• PCOS, now being renamed PMOS. The single most common cause of persistently irregular or infrequent periods, affecting roughly 1 in 10 women. Excess androgens and disrupted signalling mean follicles start but do not release an egg. See PCOS has a new name and PCOS and insulin resistance.
• Thyroid disorders. Both an underactive and an overactive thyroid alter cycle length and flow. See the thyroid and your menstrual cycle.
• High prolactin. The milk hormone suppresses ovulation; raised levels outside pregnancy come from certain medications, a benign pituitary growth, or an underactive thyroid. See prolactin in women.
• Primary ovarian insufficiency. The ovaries slow down before age 40. Periods become infrequent, then stop, often with hot flashes. It affects about 1 in 100 women under 40 and needs early diagnosis for bone and heart protection.
• GLP-1 medications. Weight loss on semaglutide or tirzepatide can restart ovulation in people who were not ovulating, which shows up as cycles changing, sometimes with unexpected fertility. See GLP-1 drugs and your cycle.

3. Structural causes

These more often change how much you bleed and how long, but they can make timing unpredictable too.

• Fibroids - benign muscle growths in the uterine wall, very common after 30. Heavy, prolonged, sometimes irregular bleeding. See uterine fibroids.
• Adenomyosis - uterine lining growing into the muscle wall. Heavy, painful periods and bleeding between them. See adenomyosis in 2026.
• Endometrial polyps - small growths on the lining that cause spotting between periods and after sex.
• Endometriosis is primarily a pain condition rather than a timing one, but it often travels with the others. See period pain vs endometriosis pain.

4. Bleeding that is not a period

Some "irregular periods" are not periods at all. Mid-cycle spotting around ovulation, breakthrough bleeding on contraception, implantation bleeding in early pregnancy, and bleeding from infection or cervical changes all get logged as extra periods and make a regular cycle look chaotic. See spotting between periods and implantation bleeding vs your period.

Medications cut across these groups: hormonal contraception (by design), some antidepressants and antipsychotics (via prolactin), blood thinners (via flow), and chemotherapy.

Symptoms that point to a cause

The pattern around the irregularity is often more informative than the irregularity itself.

Can irregular periods affect fertility?

Yes, and this is the most important reason to take a persistent pattern seriously. A regular cycle almost always means regular ovulation. An irregular one often means ovulation is late, unpredictable, or not happening. You can still conceive, but the fertile window moves and calendar-based prediction stops working.

Two practical consequences:

• If you are trying to conceive, the 2026 NICE fertility guideline advises seeking help earlier than the usual 12 months when cycles are irregular, because it signals an ovulation problem that is often treatable. See the 2026 NICE fertility guidelines. Body signs - cervical mucus, temperature, ovulation tests - work when the calendar does not; see ovulation symptoms and our beginner's guide to fertility awareness methods.
• If you are avoiding pregnancy, irregular cycles make calendar and rhythm methods unreliable. Use a method that does not depend on predicting ovulation.

There is also a health reason to act when periods are very infrequent. Without regular ovulation the uterine lining is exposed to estrogen without progesterone, and over years that raises the risk of the lining thickening abnormally. Clinicians typically recommend inducing a bleed at least every three to four months when periods are that rare, which is why the international PCOS guideline treats infrequent cycles as something to manage rather than ignore.

What a doctor will check

Bring your cycle log. A clinician assessing irregular periods will typically:

1. Take a history - cycle dates, flow, pain, weight and stress changes, medications, contraception, family history, and whether you are trying to conceive.
2. Rule out pregnancy first, whatever the cycle pattern.
3. Run blood tests - thyroid (TSH), prolactin, FSH and LH, estradiol, and androgens such as testosterone when PCOS is suspected. Timing matters: some are best drawn on day 2 to 5 of a cycle, so an irregular cycle sometimes means a random draw plus a repeat.
4. Order a pelvic ultrasound if bleeding is heavy or prolonged, if there is pain, or if PCOS or a structural cause is suspected.
5. Refer to gynaecology or endocrinology when results point to a specific condition, when you are trying to conceive, or when bleeding is heavy enough to affect daily life or iron levels.

Home hormone tests can give a preview but rarely replace this workup; see at-home hormone tests. For the full list, see what tests are done for irregular periods.

What actually helps, by cause

There is no single treatment for irregular periods because there is no single cause. What works:

• Ovulation disruptors. Address the driver. That means eating enough for your training, protecting sleep, managing the stress you can, and giving it two to three cycles. Cycles usually return once the input changes.
• Thyroid. Treating the thyroid, with levothyroxine for an underactive one or antithyroid drugs for an overactive one, usually restores the cycle within a few months.
• High prolactin. Switching the responsible medication, or a dopamine agonist, normalises prolactin in most cases, and ovulation follows.
• PCOS / PMOS. The 2023 international guideline puts the combined pill first for regulating cycles and managing androgens, with metformin as an option, especially with insulin resistance, and lifestyle change alongside. If you are trying to conceive, letrozole is the first-line ovulation induction drug. If you are having fewer than four periods a year, a course of progestin to bring on a bleed protects the lining.
• Structural causes. Fibroids, adenomyosis, and polyps have specific treatments, from hormonal IUDs and tranexamic acid for heavy flow to removal procedures. Check and correct iron.
• Perimenopause. The irregularity itself needs no treatment; troublesome symptoms have options. See hormone therapy for menopause in 2026.

A word on "regulating naturally": supplements, seed rotations, and detox plans are widely sold for this. The evidence that they change cycle length is thin; the lifestyle levers above are what the evidence supports. See how to regulate irregular periods naturally and seed cycling: what the evidence says.

When to see a doctor

Book an appointment if:

• You have missed three or more periods in a row and are not pregnant
• Cycles are consistently shorter than 21 days or longer than 35 days
• Your shortest and longest cycles differ by 20 days or more
• Periods last longer than 7 to 8 days, or you soak through a pad or tampon every hour for several hours
• You bleed between periods or after sex
• Periods that were regular have become irregular and you are under 45
• You have irregular periods and are trying to conceive
• You have no period by age 15, or periods stopped before age 40
• Irregularity comes with acne, excess hair, milky nipple discharge, or thyroid symptoms

How Femora helps

Irregular cycles are exactly where a generic 28-day assumption fails and a personal record earns its keep. Femora is built around that record:

• Real cycle statistics. Femora calculates your actual average, your shortest and longest cycle, and your variation - the regularity number a doctor asks for - from what you log, not from a default.
• Predictions with a confidence level. When cycles vary, the prediction widens and says so, instead of pretending to know the day. Cycle-synced reminders adjust the same way; see what happens to reminders when your cycle is irregular.
• Symptom and life-event logging alongside dates, so you can see whether a long cycle lines up with a stressful month, a training block, a new medication, or thyroid symptoms - and show that to a clinician as a chart rather than a memory.
• Free tools to start with: the late period calculator for a single overdue cycle and the PCOS symptom checker if the first row of the table above looks familiar.

Frequently asked questions

Is a 40-day cycle irregular? A cycle that is consistently 40 days is infrequent by FIGO's definition (over 38 days) but regular if it is 40 days every time. The NHS suggests a GP check if cycles are consistently longer than 35 days, mainly to rule out PCOS or thyroid causes. Consistently long cycles with no other symptoms are often benign; long and unpredictable cycles are more likely to be an ovulation problem. See is a 40-day cycle normal.

How many days of variation is normal? Up to 7 to 9 days between your shortest and longest cycle over a year is considered normal. Variation of 10 days or more is irregular, and 20 or more is the NHS threshold for a check.

Can you get pregnant with irregular periods? Yes. Irregular cycles usually mean ovulation is happening unpredictably, not that it never happens. Timing sex by body signs rather than calendar dates helps, and if it has not happened within a year, or sooner if cycles are very irregular, seek help. See can irregular periods affect fertility.

Do irregular periods go away on their own? The ones caused by a temporary disruption - stress, illness, travel, a training block - usually settle within two or three cycles once the cause passes. Teen and postpartum irregularity settles with time. Irregularity from PCOS, thyroid disease, or high prolactin does not resolve without treating the condition.

What is the difference between irregular and late? Late describes one period against your expected date. Irregular describes the pattern across cycles. One late period is common and rarely significant; a run of unpredictable cycles is information.

The bigger picture

An irregular cycle is not a diagnosis; it is a symptom, and a useful one. Regular ovulation depends on the brain, the thyroid, the pituitary, the ovaries, and your energy balance all cooperating, so when the cycle drifts it is usually pointing at one of them. The job is not to force the calendar back to 28 days but to find out which part of the system is asking for attention. Most of the time the answer is benign and reversible. When it is not, the conditions behind it are common, well understood, and treatable, and the earlier they are named the better.

Track your cycles, log what changed, and bring the pattern to a clinician when it crosses the lines above. That is the whole method.

Download Femora to track your cycle length, variation, and symptoms in one place: https://femora.app/download/