Why is my menstrual cycle irregular and what can I do about it?

irregular cycles

Does your cycle length change every month? Perhaps your period surprises you every time it arrives? If you’re worried that you're ‘irregular’, then we’re here to shed some light on what could be going on…

Reviewed by: Dr Bella Smith

There are lots of reasons why your period may be all over the place. But, before we dig into the details, it’s important to work out whether your menstrual cycle is actually irregular or whether the ‘28-day cycle myth’ makes you think that you are.

The 28-day cycle myth

“While many women are led to believe that their menstrual cycle should tick a 28-day box, the reality is that 28 days simply isn’t ‘always the norm’”, says NHS GP Dr Bella Smith. 

“According to scientists in a recent study , only 16% of women actually had a 28-day cycle, while a different study found that the average cycle length was actually 29 days.” 

What is a healthy menstrual cycle range?

When it comes to the range in our cycles, yet again there’s a bit of a myth that we should be exactly the same number of days every single month.

“It’s now understood that a healthy menstrual cycle can range from 21 to 35 days, and the length of your cycle can vary each month by 7 days . And this is perfectly normal,” says Bella.

“People often believe that they’re more irregular than they actually are,” continues Bella. “But the reality is that it doesn’t matter whether it’s 28 or 29 days or something  else within the range we’ve mentioned. What matters is that your menstrual cycle is regular for what you usually experience and what’s normal for you.”

How do you define an irregular cycle?

An irregular menstrual cycle is one where most of your cycles have fallen out of the normal range within the last 6 months. “If your cycle is irregular, you’ll be getting your period more often than every 21 days or less often than every 35 days,” explains Dr Bella. “And your cycle length may vary by more than 7-9 days each month.”

If this sounds like you, it’s important to see your GP, who will either be able to put your mind at rest or help you get the support you need. Here, we run through some of the possible reasons for any irregularity, then give you some ideas for what to do about it in the copy below…

  1. Stress

  2. Not eating enough

  3. Perimenopause

  4. Hormonal birth control

  5. Obesity and weight change

  6. Polycystic ovarian syndrome (PCOS)

  7. Endometriosis

  8. Depression

  9. Thyroid problems

  10.  Puberty 

  11.  Breastfeeding

What all women should do

Whatever the cause of your cycle length and symptoms, one of the best ways to understand your menstrual cycle is to start tracking it for at least three months to get a picture of your average cycle length and how or whether it changes each month. To make it easier, you can use an app, for example Jennis CycleMapping , which also syncs your workouts to suit your menstrual cycle. 

During the lockdowns we found that many of our 30-something female patients had perimenopause-like irregular cycles because stress pushed their hormones off-kilter


A 2018 study found that stress can affect the menstrual cycle, making it more irregular. “The hypothalamic-pituitary-adrenal axis (HPA axis) manages the body’s response to stress by releasing stress hormones, such as cortisol and adrenaline,” says Dr Bella. 

“One theory is that stress can put the HPA axis off-kilter, affecting the release of menstrual cycle hormones, such as luteinising hormone. Because we need this for ovulation, it may lead to irregularities in your cycle.

“During the lockdowns, for example, we found that many of our 30-something female patients had perimenopause-like irregular cycles because the stress pushed their hormones off-kilter.”

What can you do? If you think stress is sending your hormones off balance, it’s important to look at your life to see what you can take away to take the pressure off. It’s also important to schedule in time for you to do activities that relax you and bring you joy. Meditation, walking and yoga are obvious ones. But we’re all different, so it may be that rock climbing or running takes you away from it all, so make sure you prioritise that..

Not eating enough

If you’re not eating enough calories to sustain your energy output, you won’t just experience lower energy levels but your menstrual cycle can also fall out of sync. This could be through an eating issue or it could be that you’re exercising intensively but not taking in enough calories to refuel, known medically as RED-S or ‘relative energy deficiency in sport’.

“We know that energy deficiency triggers a fall in the release of follicle-stimulating hormone (FSH) and luteinising hormone LSH) – both hormones needed for a healthy menstrual cycle ,” says Dr Bella. “This leads to curbed ovulation and skipped periods. 

“If you do a lot of exercise, having a regular menstrual cycle is a sign of good health and a sign that you have a healthy balance between sport and energy intake.

What can you do? Speak to your GP about your concerns and if you think that you may have an eating disorder, reach out to the eating disorders charity Beat for support and advice. If RED-S is more likely, think about when and how you are fuelling your workouts. A sports nutritionist or dietitian can help you fine-tune your diet to fuel your exercise regime.  


If you’re over 40 and your cycle is much longer or shorter than normal, or a bit of both, you may be in perimenopause, the transition period leading up to the menopause, which is characterised by a fall in oestrogen levels. “In the early stage of perimenopause, you might find you sometimes skip a period, but in the late stage, your cycle becomes much more irregular and you can miss periods for months at a time,” says Dr Bella. 

What can you do? Speak to your GP – they can diagnose whether perimenopause or something else is causing your irregular cycle, while also offering support with other perimenopause symptoms, such as hot flushes . Tracking your cycle and symptoms can also help.

For more information on perimenopause support, check out our Perimenopause hub

Coming off hormonal birth control

While taking the pill, you won’t be having periods as ovulation stops. “As your natural cycle has been suppressed while using birth control, it can take 3-6 months for it to start up again once you come off the pill,” says Bella, “so you may have irregular cycles or no bleeding at all for 3-6 months after stopping the pill.”

“This is sometimes called ‘post-pill amenorrhoea’. If a regular cycle does not return after 6 months, this needs to be discussed with your doctor. The exception is the Mirena or copper IUD, where you are still ovulating in the background. If you’re using these forms of contraception, when you remove them, your periods should return quickly and there is a possibility of  getting pregnant straightaway.”  

What can you do? Nothing, your cycle should soon sync back into normal again.

Obesity and weight change

A 2017 study looking at around 4,600 women found that those who were obese and had recently lost or gained weight were more likely to have irregular menstrual cycles. “The scientists found that the obese women had higher levels of testosterone and lower levels of luteinising hormone, which is needed for a regular menstrual cycle,” says Dr Bella. “It’s also known that fat tissue can store hormones like testosterone and oestrogen, so this may have an impact.”

What can you do? Talk to your GP about your irregular cycles and get support in losing or gaining weight in a healthy way.

Polycystic ovarian syndrome (PCOS)

Polycystic ovarian syndrome or PCOS is caused by excess production of androgens in the ovary (particularly testosterone), which inhibits the cycle of ovarian follicles and ovulation, and symptoms can include irregular or skipped periods, weight gain, oily skin and abnormal hair growth. 

Your menstrual cycle may be giving you a sign to reach out and get support or treatment

What you can do: “See your GP,” says Dr Bella. “PCOS is diagnosed through blood tests, a scan and the clinical symptoms that you record. Tracking your cycle and symptoms can also help.

“While the cause of PCOS is not yet clearly understood, there are ways your doctor can help you manage the symptoms of PCOS. For example, treatment for acne and support with weight loss, which will help balance the hormones and make periods more regular. They will also be able to give you lifestyle advice, which is a key factor in managing PCOS symptoms.”


This condition causes womb tissue that normally grows inside the uterus to grow in other parts of the body, such as the ovaries and fallopian tubes, which in turn can cause irregular menstrual cycles. Other symptoms include severe menstrual cramping, heavy and longer periods and pain during bowel movements or when having sex. 

What can you do? See your GP. “There are medications that can help with endometriosis. However, if symptoms are moderate to severe then you they may refer you to a gynaecologist for further investigations and possible surgical intervention,” says Dr Bella.


A 2021 study reported that women with depression were more likely to have irregular menstrual cycles. Researchers found that depression caused high levels of corticotropin-releasing hormone and the stress hormone cortisol, which inhibits the release of luteinising hormone, which is needed for ovulation.

What can you do? “Yet again, it’s important to see your GP if you think this might be affecting you. It’s important to get support and treatment rather than trying to deal with it on your own. Your menstrual cycle may be giving you the signs to reach out,” says Dr Bella.

Thyroid problems

If your doctor has diagnosed an underactive thyroid gland (hypothyroidism) or an overactive thyroid gland (hyperthyroidism), it’s likely that your symptoms include irregular periods. “This is because thyroid hormones have an impact on the reproductive hormones,” says Dr Bella. 

“With an underactive thyroid gland (called: hypothyroidism), your body doesn’t produce enough of the thyroid hormone thyroxine, which can lead to missed periods, periods occurring more often than 21 days and heavy bleeding. For an overactive thyroid gland (called: hyperthyroidism), your body produces too much thyroxine and this can cause more irregular periods.” 

Other symptoms of hypothyroidism are:

  • Tiredness

  • Weight gain

  • Cold sensitivity

  • Muscle aches

  • Dry skin

  • Brittle hair and nails

Other symptoms of hyperthyroidism include:

  • Anxiety

  • Hyperactivity

  • Insomnia 

  • Diarrhoea

  • Thirst

  • Tiredness

  • Heat sensitivity

What can you do? See your GP for diagnosis, medication and support with thyroid problems. If you are found to have thyroid disease, this will need to be monitored closely to start with and then annually once stable and you are on medication.


After starting your periods, it can take around 3 years for your body to settle into the rhythm of a regular cycle.

What can you do? Just monitor it. “If a teenager is fit and well, it is absolutely normal for her menstrual cycle to seem irregular and even skip a few months in a row,” says Dr Bella. “However, do see your GP if you’re a teenager and your periods had become regular but are now irregular or they’re very heavy or painful.”


A hormone in breast milk called prolactin curbs ovulation – which means you won’t have a period in the early stages of breastfeeding. Ongoing, this is more common if you are breastfeeding exclusively. If you are topping up with formula, you are more likely to have the odd period.

What can you do Sit tight, your cycle will settle down again soon enough. “Just be aware that you can still get pregnant,” says Dr Bella.

Dr Bella is one of the experts behind  The WellHQ

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