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Irregular periods: why is my period late?

3 women laughing linking arms. Happy expressions
Irregular periods are common and can include cycles that are too long, too short, or characterised by heavy or prolonged bleeding. Learn more in our article.
Anyone who gets a menstrual cycle will have probably experienced an irregular period at some point in their life. And while it can be slightly alarming when it happens, in most cases, there’s nothing to worry about.

In this article, we define what an irregular period is, explore what some of the causes of irregular periods can be, and highlight the importance of tracking your cycle to stay on top of your menstrual health.

What is a ‘normal’ menstrual cycle?

You’ve probably heard that a ‘normal’ menstrual cycle is 28 days. But the truth is that very few people who menstruate actually have 28-day cycles.

In fact, a recent study proved this, finding that – of more than 600,000 menstrual cycles – just 13% of those were 28 days long.1 Therefore, an expected menstrual cycle is said to actually range from anywhere between 21 and 35 days. 2 

Your menstrual cycle also has four distinct phases: 
  1. menstruation: day one of your menstrual cycle is the first day of your period. A typical period can last between two and seven days, with the heaviest bleeding occurring on the first two days3 
  2. follicular phase: during this phase, your body releases follicle-stimulating hormones (FSH), causing the follicles in your ovaries to grow. Meanwhile, the hormone oestrogen is released to thicken the womb’s lining. This phase begins on day one of your cycle (overlapping with your period) and ends with ovulation (when an egg is released from one of your ovaries)4 
  3. ovulation: when a mature egg is released from the ovary, ovulation happens. This is triggered by a surge of luteinising hormone (LH). Ovulation typically occurs 12‒16 days before your period4 
  4. luteal phase: after ovulation, the egg travels along the fallopian tubes to the uterus. If it meets a sperm along the way, it might become fertilised and implant onto the now-thick lining of the womb. If the egg isn’t fertilised, hormone levels drop, and the lining of the uterus starts to shed, triggering your period

What are irregular periods?

Irregular periods can be anything that doesn’t fit the ‘normal’ expected pattern of a 21‒35-day cycle. 

Some examples of irregular periods include:5
  •  periods that occur more than 35 days apart or less than 21 days apart
  • menstrual bleeding that’s either much heavier or lighter than usual 
  • not having a period for three consecutive months
  • periods with irregular cycles that vary more than seven to nine (for example, one month your menstrual cycle is 24 days long and the following month it’s 34 days long) 
  • bleeding that lasts longer than seven days 
In general, your menstrual cycle can be a good indication of your overall health. But the reality is that irregular periods are very common and, in most cases, aren’t anything to be alarmed about.6

What are the causes of irregular periods?

Some reasons for irregular cycles may be due to recent changes in your lifestyle, whereas others might be due to an underlying medical condition.

Lifestyle factors 

It’s perfectly normal for your period to vary slightly from cycle to cycle. Your menstrual cycle is highly responsive to changes in your daily routine. Stress, in particular, can affect your body’s delicate balance.7

Some lifestyle factors that may affect your cycle include:6
  • stress and anxiety 
  • changes in weight (either weight loss or weight gain)
  • excessive exercise or sports
  • changes to your diet

Common life stage causes 

During certain stages of life, you also have a higher chance of experiencing irregular periods. 

 For example, you might get an irregular period due to:6 
  • puberty: it can take up to two years for oestrogen and progesterone levels to regulate. During this time, irregular periods are very common8
  • starting menopause: one of the tell-tale signs of perimenopause (the time leading up to the menopause) is irregular periods9
  • certain types of contraception: both the contraceptive pill and coil can change your ‘normal’ menstruation cycle
  • pregnancy: a late period can be the first sign of pregnancy. And, even in early pregnancy, there can be some irregular bleeding

Underlying medical conditions

In some cases, irregular periods can also be caused by an underlying medical condition. These include: 
  • endometriosis: this condition occurs when endometrial tissue grows outside the womb on the fallopian tubes or the ovaries. It’s characterised by heavy bleeding, painful periods, and bleeding between periods10
  • polycystic ovary syndrome (PCOS): this causes the ovaries to produce high levels of male hormones (androgens) which interfere with regular menstruation11
  • thyroid disorders: your thyroid helps regulate many systems in your body, including the timing of your ovulation and period. Either too much or too little thyroid hormone can cause irregular cycles12

Signs that your period is coming 

If your periods are unpredictable, it can be helpful to know which signs and symptoms to look out for that indicate your period is on its way.

Premenstrual syndrome (PMS) is the collective name given to the wide range of emotional and physical symptoms experienced in the weeks before a menstrual period.

Common PMS symptoms include:13,14
  • mood swings 
  • irritability
  • tiredness
  • sleeping difficulties
  • headaches
  • bloating 
  • cramping 
  • breast tenderness
  • headaches
  • altered sexual desire
  • spots and greasy hair
  • changes in appetite and food cravings
PMS symptoms affect people in different ways. So if you experience any symptoms that are especially painful or impact your ability to perform day-to-day activities, consult your GP.

Menstrual period trackers

If you’re one of the many individuals who experience irregular periods, this lack of predictability can be understandably distressing.

However, one of the best ways to better understand your body is by tracking your cycle either in your diary or on a period tracking app, to see which trends emerge over time. 

A period tracking app is a great tool to learn your body’s unique patterns and help manage your moods.15 The insights it can provide will also help you if you want to talk to your doctor about your irregular cycle or any other concerns you might have.

For example, you should make a GP appointment if you have painful periods, bleeding that lasts more than seven days, very heavy bleeding, sudden drastic changes in your period (especially if you’re normally regular), or you’d like to get pregnant.6

The final say

Irregular periods and small variations in your menstrual cycle are completely normal. However, irregular cycles that are either too long, too short, or characterised by very heavy or prolonged bleeding could be a sign of a bigger issue. 

There are many reasons you might have an irregular cycle – from lifestyle changes like stress and excessive exercise to medical conditions like PCOS and endometriosis.

Period-tracking apps can also be especially useful for gathering insights into your menstrual health and sharing important personal information with your GP when required.

Sources

  1. Bull, J., et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine. 2019;27;2(1). Available from: https://www.nature.com/articles/s41746-019-0152-7.
  2. American College of Obstetricians and Gynecologists. Abnormal Uterine Bleeding [Internet]. [cited 2024 Feb 22]. Available from: https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding.
  3. NHS. Periods [Internet]. [cited 2024 Feb 22]. Available from: https://www.nhs.uk/conditions/periods/.
  4. American Pregnancy Association. What is Ovulation? [Internet]. [cited 2024 Feb 22]. Available from: https://americanpregnancy.org/getting-pregnant/infertility/understanding-ovulation/.
  5. American College of Obstetricians and Gynecologists. Heavy and Abnormal Periods [Internet]. 2020 [cited 2024 Feb 22]. Available from: https://www.acog.org/womens-health/faqs/heavy-and-abnormal-periods.
  6. NHS. Irregular periods [Internet]. 2017 [cited 2024 Feb 22]. Available from: https://www.nhs.uk/conditions/irregular-periods/.
  7. Bertone-Johnson, E., et al. Early Life Emotional, Physical, and Sexual Abuse and the Development of Premenstrual Syndrome: A Longitudinal Study. Journal of Women’s Health. 2014;23(9):729–39. Available from: https://pubmed.ncbi.nlm.nih.gov/25098348/.
  8. NHS Inform. Irregular periods [Internet]. [cited 2024 April 10]. Available from: https://www.nhsinform.scot/healthy-living/womens-health/girls-and-young-women-puberty-to-around-25/periods-and-menstrual-health/irregular-periods/
  9. NHS. Symptoms - Menopause [Internet]. [cited 2024 Feb 22]. Available from: https://www.nhs.uk/conditions/menopause/symptoms/.
  10. Endometriosis Foundation of America. Endometriosis Symptoms: Abnormal Periods [Internet]. [cited 2024 Feb 22]. Available from: https://www.endofound.org/abnormal-periods.
  11. World Health Organization. Polycystic ovary syndrome [Internet]. [cited 2024 Feb 22]. Available from: https://www.who.int/news-room/fact-sheets/detail/polycystic-ovary-syndrome. ‌
  12. Jacobson, MH., et al. Thyroid hormones and menstrual cycle function in a longitudinal cohort of premenopausal women. Paediatric and Perinatal Epidemiology. 2018;32(3):225–34. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5980701/.
  13. Gudipally, P., et al. ‘Premenstrual Syndrome’. StatPearls, StatPearls Publishing, 2023. https://www.ncbi.nlm.nih.gov/books/NBK560698/.
  14. NHS. PMS (premenstrual syndrome) [Internet]. [cited 2024 Feb 23]. Available from: https://www.nhs.uk/conditions/pre-menstrual-syndrome/.
  15. Levy, J., et al. “A good little tool to get to know yourself a bit better”: a qualitative study on users’ experiences of app-supported menstrual tracking in Europe. BMC Public Health. 2019;19(1). Available from: https://bmcpublichealth.biomedcentral.com/articles/10.1186/s12889-019-7549-8. ‌
 
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