Reviewed July 2019

Here’s how to determine the cause of your irregular periods

Here's how to determine the cause of your irregular periods 1200400

Every woman’s menstrual cycle is different. Some women find that their menstruation runs like clockwork and follows a typical 28 day cycle, while others have cycles that are longer or shorter than this. Cycles can last anything from 24 to 35 days, with menstrual periods lasting anything from two days to a week. Due to the variation between different women there isn’t a definition for a ‘normal’ menstruation cycle.

An irregular menstruation is any period that is abnormal compared to the last few menstrual cycles for that individual person. Irregular periods can take many forms, for example a change in the timing meaning that it is late, early, or even absent (amenorrhea). It may be a difference in the amount of bleeding which can be scant or heavy. The number of periods may also be irregular, such as having more than one period in a month. When menstruation becomes ‘out of sync’, this is thought to be caused by an imbalance between the hormones oestrogen and progesterone. This is not uncommon in the first few years after puberty, or before menopause. Once periods are established, woman may experience an irregular period now and again and it is not necessarily anything to worry about, although it is important to figure out what may be causing this irregularity. This article aims to explain some common causes of irregular periods.


There are lifestyle factors that can upset the hormonal balance, causing irregular bleeding. Stress is one of the most common causes. When we are stressed (emotionally or physically) our bodies release a hormone called cortisol. This has a direct impact on oestrogen and progesterone which in turn affects the timing and flow of menstruation.1

Similarly, excessive weight loss or weight gain will cause the same hormone imbalance that affects menstruation. Women with anorexia nervosa or bulimia are likely to have irregular periods, as maintaining an ‘ideal’ body mass index of 18 to 25Kg/m helps with having a regular menstrual cycle.2 Excessive exercise or sudden changes in weight can cause the hormone called leptin, produced by fatty tissue, to drop. Along with other hormonal drops this can also disrupt the regularity of periods.3 It’s interesting to note that many professional runners, swimmers, and ballet dancers may menstruate infrequently due to their lifestyles.

Other habits such as smoking4 and caffeine5 can also cause irregular periods. This is because the body reacts to smoking and caffeine like a stress response: cortisol and other stress hormones are released, affecting the menstruation cycle. Taking certain drugs may also affect periods, including ibuprofen when taken in high doses.6 This is because it can stop the functioning of prostaglandin, a lipid that helps relax the smooth muscles of the womb during menstruation.


Using the oral contraceptive pill can cause breakthrough bleeds when the body is first getting used to the hormones. These periods are usually lighter or shorter than normal and stop after the first few months of starting contraception. Contraceptive pills can also cause spotting between periods, whilst intrauterine devices, such as the coil, can cause heavy or painful periods. Spotting and breakthrough bleeding occurs because there is relatively little or no oestrogen, which when present helps to stabilise the womb lining.7 When there are high levels of progesterone relative to oestrogen, such as when taking the mini-pill or a small dose of oestrogen (taking low oestrogen pills), the uterus lining can shed a little, causing spotting.


Pregnancy stops menstruation because the uterine lining will not shed when there is an implanted fertilised egg. Women who are pregnant do not usually bleed, therefore you must see a doctor if there is any bleeding during pregnancy as this may be caused by early miscarriage or ectopic pregnancy. Breastfeeding causes irregularity in periods because the suckling of breastfeeding releases hormones: oxytocin and prolactin. These hormones, in turn, stop the release of other hormones in the brain called gonadotrophins.8 Without gonadotrophins, the womb lining does not thicken and there is no shedding. Absence of periods during breastfeeding is known as lactation amenorrhea. Regular periods do return, generally after six months of breastfeeding.

Polycystic ovary syndrome (PCOS)

One in five women are thought to have PCOS in the UK.9 Irregular periods combined with excessive hair growth, weight gain, and acne can be signs of PCOS. Small, fluid-filled sacs (cysts) develop in the ovaries which affects how often the eggs are released, and the hormones are imbalanced, with higher levels of testosterone. The exact cause is unknown but PCOS often tends to run in families. The treatment options vary depending on the symptoms, but can include weight loss, the contraceptive pill, or a drug that encourages ovulation such as clomifene.

Thyroid disorders

If irregular periods are associated with a lump in the neck, palpitations, and anxiety, this can be a sign of an overactive thyroid. The thyroid gland is found in the neck and produces hormones that help keep the body’s metabolism in check. If you are concerned that you may have thyroid disorder contact your GP, who can take a blood test that will check levels of the thyroid hormone in the blood.

Structural problems

If there is severe lower abdominal pain or heavy bleeding associated with irregular periods then this may be due to problems with the womb such as fibroids (non-cancerous growths), cysts, polyps (benign growth with a stalk), and endometriosis (tissue that behaves like womb lining outside the womb). The causes of these conditions are unknown and some women may not experience any symptoms at all. If you have any unusually painful or heavy periods, pain during or after sex, bleeding between periods, or difficulty in getting pregnant then it’s important to see your GP. The GP may do a pelvic examination and order an ultrasound to evaluate the structure of the womb and ovaries.

Having regular periods are a good marker of a healthy balance of the hormones oestrogen and progesterone. Therefore, whenever there is a stray from the usual routine, it is worth checking to see if the above causes may be the reason. There are certain situations where it is especially important to seek medical advice, including situations where you’re bleeding between periods, bleeding during or after sex, and experiencing particularly heavy, long-lasting, or painful periods as these may indicate structural problems. Furthermore, women’s bodies are not designed to be without any periods at all. So, providing pregnancy is not a cause, if periods are absent for more than six months, it is important to visit your GP or gynaecologist for advice.

Last updated July 2019
Next update due 2021

Dr. Diana Chiu, MBChB (Hons) MRCP PGCERT (Med Ed) PhD

Diana received her medical degree, with honours, from the University of Manchester. She then went on to receive basic and specialist medical training within the north west of England. She carried out in-depth research in medicine and was awarded a PhD in 2016. Currently, she is finishing her medical training at a large teaching hospital, and one of her greatest interests is medical education. She is an advanced life support instructor and writes regularly for post-graduate examination websites, and also holds a PGCERT in medical education with distinction.

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  1. Prasad, S., et al., Impact of stress of oocyte quality and reproductive outcome, Journal of Biomedical Science, March 2016, vol 23, no.36
  2. Vale, B., et al., Menstruation disorders in adolescents with eating disorders — target body mass index percentiles for their resolution, Einstein (Sao Paulo), April-June 2014, vol 12, no. 2., pp 175-180
  3. Fontana, R., and Della Torre, S., The deep correlation between energy metabolism and reproduction: a view on the effects of nutrition for women fertility, Nutrients, February 2016, vol 8, issue 2, pp 87
  4. Bae, J., et al., Factors associated with menstrual cycle irregularity and menopause, BMC Women’s Health, February 2018, vol 18, no. 36
  5. Mahmoud, A.Z.B., et al., Association between menstrual disturbances and habitual use of caffeine, Journal of Taibah University Medical Sciences, December 2014, vol 9, issue 4, pp 341-344
  6. Lethaby, A., et al., Non-steroidal anti-inflammatory drugs for heavy menstrual bleeding, The Cochrane Database of Systematic Reviews, 2013, [online], (accessed 4 July 2019)
  7. Page Wright, K., and Johnson, J.V., Evaluation of extended and continuous use oral contraceptives, Therapeutics and Clinical Risk Management, October 2008, vol 5, no. 5, pp 905-911
  8. World Health Organisation (WHO), ‘The physiological basis of breastfeeding’, WHO, Infant and Young Child Feeding: Model Chapter for Textbooks for Medical Students and Allied Health Professionals, [online], (accessed 5 July 2019)
  9. NHS Choices Health A-Z, ‘Polycystic ovary syndrome’, [website], 2016,, (accessed 20 March 2017).

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