Your period's late. You've checked the calendar three times, maybe taken a test, and now you're sitting here wondering what on earth your body is playing at.

Here's the short version: most late periods come down to one of a handful of causes, pregnancy, stress, a significant shift in weight or exercise, a thyroid issue, perimenopause, coming off or starting hormonal contraception, or polycystic ovary syndrome (PCOS). One missed period on its own is rarely a crisis. But if it keeps happening, that's your body trying to tell you something, and it deserves proper investigation, not a shrug and a "just relax."

This guide walks through every common cause of a late or missed period, how to tell them apart, why PCOS deserves a closer look if your cycles are consistently unpredictable, and exactly when it's time to get things checked out properly. We'll also cover how to track your cycle in the meantime, so if you do end up in a GP appointment, you're walking in with information instead of guesswork.

What Counts as a "Late" or "Missed" Period?

Short answer: A period is usually "late" once you're past day 35 of your cycle, and "missed" if it doesn't turn up at all for one or more cycles.

A normal menstrual cycle is counted from the first day of one period to the first day of the next, and anywhere from 21 to 35 days is considered typical.^[1] The average sits around 28 days, but "average" is doing a lot of heavy lifting there. Plenty of people with perfectly healthy cycles run shorter or longer than that, and a bit of month to month wobble, a few days here or there, is completely normal.

So when does "a bit late" tip into "actually missed"? Here's a rough guide:

• Late: Your period hasn't arrived by day 35 to 40 of your cycle, counting from the first day of your last period.
• Missed: No period at all for a full cycle length, so roughly 35 plus days with nothing showing.
• Irregular: Your cycle length varies by more than 7 to 9 days from month to month, even if a period does eventually arrive each time.
• Absent (amenorrhoea): No periods for three months or more, which is when it's worth booking in with your GP if you're not pregnant.

A single late period, especially after a stressful month, an illness, or travel, usually isn't something to panic about. Your body is trying to tell you something with a pattern, not necessarily with one off-cycle. The rest of this article is about spotting that pattern and knowing what it might mean.

Could I Be Pregnant?

Short answer: If you're sexually active and your period is late, take a home pregnancy test first. It's the fastest way to rule this cause in or out.

We know, this is the obvious one. But it's obvious for a reason: pregnancy is the single most common cause of a missed period, so it's worth ruling out before you go digging through hormone theories. A home pregnancy test is most accurate from the first day of your missed period onwards, and testing with your first morning wee gives the most concentrated (and therefore most reliable) result.

If you get a negative test and your period still hasn't shown up a week or so later, it's worth testing again, since testing too early can occasionally give a false negative. If you've had two negative tests and no period, the cause is almost certainly something else on this list, and it's time to look at the other usual suspects.

What Else Can Delay Your Period Besides Pregnancy?

Short answer: Stress, weight changes, over-exercising, thyroid issues, perimenopause, contraception changes, and PCOS are the next most common causes.

Once pregnancy is off the table, the list of possible causes widens quite a bit. Your cycle is run by a delicate hormonal feedback loop between your brain (the hypothalamus and pituitary gland) and your ovaries, and almost anything that puts your body under real strain can throw that loop off course for a cycle or two.

Stress

This one surprises people, but it's genuinely one of the most common causes of a late period. Chronic or acute stress raises cortisol, and cortisol can suppress the hormones responsible for triggering ovulation. No ovulation, no period, or a significantly delayed one. A big life event, a bereavement, a new job, exam season, a house move, any of it can be enough to push your cycle off track for a month.

Significant Weight Change

Both rapid weight loss and rapid weight gain can delay or stop your periods. Your body needs a certain amount of stored energy to keep your reproductive hormones running normally, so a sudden calorie deficit, an eating disorder, or an overly restrictive diet can switch ovulation off as a kind of energy-conservation measure. On the flip side, significant weight gain can also disrupt cycles, partly through its effect on insulin and oestrogen levels.

Over-Exercising

Training hard, especially combined with under-eating, can cause what's sometimes called hypothalamic amenorrhoea, where your brain essentially puts reproduction on pause because it doesn't have the energy reserves to "spend" on it. This is common in endurance athletes, dancers, and anyone who's recently ramped up training volume without matching it with enough food.

Thyroid Conditions

Your thyroid gland and your reproductive hormones are closely linked, so both an underactive thyroid (hypothyroidism) and an overactive one (hyperthyroidism) can cause missed, light, or heavy periods. Thyroid issues often come with other symptoms too, like fatigue, weight changes that don't match your eating habits, hair thinning, or feeling constantly too hot or too cold. A simple blood test can check this.

Perimenopause

If you're in your 40s (or occasionally late 30s), irregular or missed periods can be an early sign of perimenopause, the transition phase before menopause where oestrogen and progesterone start to fluctuate. Cycles often become shorter, then longer, then unpredictable, sometimes alongside hot flushes, night sweats, sleep disruption, or mood changes. Nobody warns you about this bit, which is honestly one of the more frustrating gaps in women's health education.

Coming Off or Starting Hormonal Contraception

If you've recently stopped the pill, had a contraceptive implant or coil removed, or come off the injection, it can take your body a few months to resume its own natural cycle, this is sometimes called post-pill amenorrhoea. Equally, starting a new hormonal contraceptive, or switching between methods, can cause irregular bleeding or a missed period while your body adjusts.

Less Common Causes Worth Knowing

A shorter list, but worth a mention: certain medications (including some antidepressants, antipsychotics, and chemotherapy drugs), chronic illnesses (such as coeliac disease or type 1 diabetes), and structural issues with the uterus or cervix can all delay or stop periods. High prolactin levels, sometimes linked to a small, usually harmless pituitary growth, can also interfere with ovulation and are picked up on a routine blood test. If none of the more common causes fit your situation, this is worth flagging to your GP rather than trying to self-diagnose from a search engine at midnight.

Possible Cause	Typical Accompanying Symptoms
Pregnancy	Breast tenderness, nausea, fatigue, positive pregnancy test
Stress	Sleep disruption, tension, appetite changes, one-off delayed cycle
Significant weight change	Rapid weight shift, fatigue, hair thinning, light or absent periods
Over-exercising	High training load, low energy availability, absent periods
Thyroid conditions	Fatigue, unexplained weight change, temperature intolerance, hair changes
Perimenopause	Hot flushes, night sweats, sleep and mood changes, cycles aged 40 plus
Contraception changes	Recently started or stopped hormonal contraception
PCOS	Acne, excess facial or body hair, scalp hair thinning, weight changes, long-term irregular pattern

Could PCOS Be Behind My Missed Periods?

Short answer: Yes, PCOS is one of the most common causes of irregular or missed periods, affecting around 1 in 10 women of reproductive age.

This is the part we know best, so let's spend some proper time here. Polycystic ovary syndrome (PCOS) is a hormonal condition affecting an estimated 1 in 10 women of reproductive age in the UK, and it's one of the leading causes of ovulation-related infertility.^[2] If your periods have been unpredictable for a while rather than just this once, PCOS deserves a proper look.

What's Actually Going On

PCOS is driven by two main hormonal patterns, and they tend to feed into each other:

• Higher androgen levels. "Androgens" are hormones like testosterone that everyone produces in small amounts, but in PCOS they're often elevated. This can interfere with the signal your ovaries need to release an egg each month, which means ovulation doesn't happen reliably, and without ovulation, a period either doesn't arrive or arrives late.
• Insulin resistance. Up to 70 percent of people with PCOS have some degree of insulin resistance, meaning their cells don't respond to insulin as efficiently, so the body produces more of it to compensate.^[3] Higher insulin levels can, in turn, push androgen production up even further, which is part of why PCOS symptoms tend to cluster together rather than showing up in isolation.

The research on this is actually pretty clear: this androgen and insulin relationship is central to why ovulation becomes so unpredictable in PCOS, not just an occasional late period but a genuinely erratic pattern that can stretch cycles out to 40, 60, or even 90-plus days, or stop them altogether for months at a time.

How Is PCOS-Related Irregularity Different From an Occasional Late Period?

A late period caused by stress or a rough month usually happens once, then your cycle settles back to its normal rhythm. PCOS-related irregularity tends to look different:

• It's a long-term pattern, not a one-off, often going back to your teenage years
• Cycles are frequently longer than 35 days, or unpredictable in a way that makes tracking difficult
• It's usually accompanied by other symptoms: acne (especially along the jawline), excess hair growth on the face or body, thinning hair on the scalp, or weight that's hard to shift despite consistent effort
• Periods, when they do arrive, can be unusually light or unusually heavy

If that combination sounds familiar, you're not imagining it. Too many people spend years being told their irregular cycles are "just how their body is," or being handed a prescription for the pill to mask the symptoms without anyone investigating the root cause. Root cause, not symptom suppression, is genuinely the more useful place to start, because understanding what's driving the irregularity gives you far more to work with than simply covering it up.

Getting a Diagnosis

In the UK, PCOS is usually diagnosed using the Rotterdam criteria, which requires at least two of the following three features: irregular or absent ovulation, clinical or blood-test signs of higher androgens, and polycystic-appearing ovaries on ultrasound.^[4] You don't need all three, and you don't need visible cysts on a scan to have PCOS, which surprises a lot of people. Your GP can run blood tests to check your hormone levels and may refer you for a pelvic ultrasound.

It's worth knowing that PCOS symptoms can look quite different from person to person. Some people have textbook irregular cycles and visible cysts on a scan but no acne or excess hair. Others have all the androgen-related skin and hair symptoms with a cycle that's only mildly irregular. This is part of why it's so often missed or misdiagnosed, and why a GP who properly investigates rather than pattern-matching to the most obvious presentation makes such a difference.

Here's what we'd say to anyone waiting on that appointment: your diagnosis is a starting point, not a verdict. It's a name for a pattern your body has been showing you, and once you have it, you can actually build a plan around it instead of guessing in the dark. This is what we wish someone had told us earlier, hormonal literacy isn't complicated, it's just rarely taught.

If irregular cycles turn out to be linked to PCOS, a daily myo-inositol supplement like Myoplus may help support hormonal balance alongside the basics of good sleep, regular movement, and keeping stress in check. It's not a magic fix, but it gives your body what it's often missing.

When Should I See a GP About a Missed Period?

Short answer: See your GP if you've ruled out pregnancy and you've missed periods for 3 months or more, or if a new pattern feels concerning.

This is the bit we want you to take seriously: persistent irregularity is not something you need to "wait and see" about indefinitely. Book a GP appointment if:

• You've had a negative pregnancy test and no period for 3 months or more
• Your cycles have suddenly become much longer, shorter, or more unpredictable than your usual pattern
• You're experiencing missed periods alongside acne, excess hair growth, hair thinning, or unexplained weight changes
• You've never had a period by age 16 (this is worth investigating on its own)
• You're trying to conceive and your cycles are irregular
• You have any pain, unusual discharge, or bleeding between periods alongside the irregularity

If you've ever been told to "just relax" or "lose a bit of weight" and come back if it's still a problem, you're not alone, and you're allowed to push for more. A pattern of missed or irregular periods is a legitimate medical symptom that deserves blood tests, not a shrug. Your GP can check your thyroid function, hormone levels (including androgens, LH, and FSH), and refer you for an ultrasound if PCOS or another structural cause is suspected.

How Can I Track My Cycle Before My GP Appointment?

Short answer: Use a cycle tracking app for at least 2 to 3 months and note cycle length, symptoms, and any lifestyle changes before your appointment.

Walking into a GP appointment with actual data makes a huge difference to how quickly you get taken seriously and how accurately you get assessed. A period tracking app (there are plenty of good free ones) makes this far easier than trying to remember dates off the top of your head.

What to Track

• Cycle length: the number of days from the first day of one period to the first day of the next
• Flow: light, normal, or heavy, and whether this has changed recently
• Symptoms: acne, excess hair growth, hair thinning, mood changes, pain, bloating, breast tenderness
• Weight changes: any significant gain or loss over the past 6 to 12 months
• Lifestyle factors: major stress events, changes in exercise routine, travel, illness
• Medications: anything you've started, stopped, or changed, including contraception

What to Bring to Your Appointment

A simple written or printed summary of the above, covering at least 2 to 3 months if you have it, gives your GP something concrete to work from. It's also worth writing down your questions beforehand, it's easy to forget them once you're in the room, and a written list means you walk out with actual answers instead of a vague "let's monitor it." Questions worth having ready include: what blood tests are you running and why, could this be PCOS or a thyroid issue given my symptoms, and what happens if my periods are still irregular in three months.

Frequently Asked Questions

How late can a period be before I should worry?
A few days to a week late is usually nothing to worry about. If you're more than 5 to 7 days past your expected date, a pregnancy test is a sensible first step, and if periods are absent for 3 months or more, it's worth seeing your GP.

Can stress delay your period?
Yes. Stress raises cortisol, which can interfere with the hormones needed to trigger ovulation, and no ovulation typically means a late or missed period. A single stressful month is a common, harmless cause of one delayed cycle.

Can a missed period mean PCOS?
It can, especially if irregular cycles have been a long-term pattern rather than a one-off, and particularly alongside symptoms like acne, excess hair growth, or weight changes. PCOS affects around 1 in 10 women and is one of the most common causes of ongoing irregular periods.

What tests will a GP do for irregular periods?
Typically a blood test to check thyroid function and hormone levels (including androgens, LH and FSH), and sometimes a pelvic ultrasound if PCOS or a structural cause is suspected. Your GP may also ask about your cycle history, weight changes, and lifestyle.

Does losing weight bring periods back?
For some causes, yes, a return to a sustainable weight and energy balance can help regular ovulation resume, particularly where periods stopped due to rapid weight loss or over-exercising. For PCOS, weight changes can help some people's cycles become more regular, though it's not a guarantee and isn't the whole story for everyone.

Can supplements help regulate periods?
Some supplements, including myo-inositol, may help support hormonal balance in people with PCOS related irregularity, alongside sleep, movement, and stress management. Supplements support the basics, they don't replace a proper diagnosis or medical advice.

Is it normal for period length to change as I get older?
Some variation is normal throughout your reproductive years, but a noticeable, sustained shift in your 40s is often an early sign of perimenopause, when oestrogen and progesterone naturally start to fluctuate.

Should I stop tracking once my period returns?
No, keep going. Ongoing tracking helps you and your GP spot patterns over time, and gives you a head start if irregularity returns or changes down the line.

Conclusion

A late or missed period is your body flagging that something in its usual rhythm has shifted, sometimes for a completely harmless reason, sometimes for something worth properly investigating. Pregnancy, stress, weight changes, over-exercising, thyroid issues, perimenopause, contraception changes, and PCOS cover the vast majority of cases, and most of them are manageable once you know what you're dealing with.

If this has been a one-off, there's a good chance it'll sort itself out. If it's been a pattern, particularly alongside other symptoms, that's worth taking to your GP rather than waiting it out. Hormonal literacy isn't complicated, it's just rarely taught, and understanding your own cycle is one of the most useful pieces of health knowledge you can have.

Further Reading

• What Is PCOS? A Complete Guide
• PCOS and Insulin Resistance Explained
• Getting a PCOS Diagnosis: What to Expect

References

1. NHS. "Irregular periods." nhs.uk/symptoms/irregular-periods
2. NHS. "Polycystic ovary syndrome (PCOS)." nhs.uk/conditions/polycystic-ovary-syndrome-pcos
3. NICE Clinical Knowledge Summaries. "Polycystic ovary syndrome." cks.nice.org.uk/topics/polycystic-ovary-syndrome
4. NICE Clinical Knowledge Summaries. "Amenorrhoea." cks.nice.org.uk/topics/amenorrhoea

This article is for general information only and does not replace advice from your GP or a qualified healthcare professional. Food supplements should not be used as a substitute for a varied and balanced diet. If you're pregnant, breastfeeding, taking medication or managing a diagnosed health condition, check with a healthcare professional before starting a new supplement.

References