What cause irregular periods (menstrual bleeding)?

Two young women on a red background

Let’s start with the most important thing: irregular menstruation can be totally normal, so breathe a sigh of relief. Your period rarely shows up when you think it will – maybe for you it starts a week early, you bleed more heavily than usual, or you miss your period entirely – you’re definitely not alone. Having an irregular period cycle is surprisingly common. They can happen in about 30% of women in their reproductive years i.e., the time during which you have periods and are physically capable of getting pregnant.

Those reproductive years span your teenage years – or whenever you first get your period – to the wind-down of your menstrual cycle, which happens during pre-menopause in your mid-to-late forties. And it’s when things are starting up and winding down that you’re most likely to deal with an irregular menstrual bleeding (teenage irregular periods), which makes sense, since there’s a lot going on in your body that can upset your schedule. Really, your body is just figuring out its new schedule, which can mean your period comes early, late, or not at all.

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What is an irregular period and how long should a period last?

Why are my period irregular? Menstrual cycles are different for everyone, and the timeline varies even once you’re on a regular schedule. While the average length of a menstrual cycle tends to be around 28 days, that’s definitely not the case for everyone – and it also isn’t the same exact timing every single month. Irregular periods are totally normal – just think of it as a monthly guessing game, the women’s health edition.

The exact time between bleeding can be anywhere from 21 to 35 days – it just depends on your body. Generally, your period is considered late when it comes after the 35-day mark. If your period falls beyond this range more than once or twice, it can be a sign of something else going on. It’s a good idea to seek medical advice if your menstrual cycle is less than 21 days or more than 35 – or if you experience other symptoms of irregular periods (for example, menstruation may occur earlier - find out the reasons), such as heavy bleeding or pain. Your doctor can help you get to the bottom of it.

Tracking your period can also give you a better sense of how long your cycle is. It’s really easy – just use our free period tracker to calculate when your next period will be.

Types of irregular periods

Regular menstruation is one sign of a woman's overall good health and hormonal balance. What do irregular periods mean? Menstrual disorders can be divided according to the cause of their onset and their nature. The most common changes in the balance of menstruation concern the regularity of cycles, the abundance of bleeding and the presence of increased pain.

Absence of menstruation (amenorrhea)

Absence of menstruation (amenorrhea) is a primary or secondary menstrual disorder. Primary amenorrhoea is when a girl has not had her first menstrual period, even though she should have had one due to her age. The age of 14 years is taken as such a limit. Secondary amenorrhoea is when menstrual bleeding stops in a normally menstruating woman.

Absence of menstruation may be a consequence of a systemic disease or a dysfunction on the part of the reproductive system. The most common cause of amenorrhoea in a woman of reproductive age is pregnancy. Other causes are anatomical abnormalities, e.g. complete hymen overlap, absence of a reproductive organ component, genetic diseases, hormonal disorders (e.g. increased prolactin levels - hyperprolactinaemia, primary hypothyroidism). In some systemic diseases, especially autoimmune diseases, premature cessation of ovarian function - POF - can occur, resulting in premature menstrual disappearance. Other causes should not be overlooked, such as eating disorders, competitive sports, stress or taking certain medications that can affect the hormonal balance.

Haemorrhagic periods (menorrhagia)

Another type of menstrual dysfunction is haemorrhagia (menorrhagia), in which there is increased blood loss during menstruation or intermenstrual bleeding (more than 80 ml). In this dysfunction, bleeding can last for more than seven days. They are most often caused by anatomical abnormalities of the uterus: polyps, myomas, adenomyosis, less commonly endometrial cancer, as well as other causes such as ovarian dysfunction, coagulation disorders, functional changes in the endometrium or taking anticoagulants.

Painful menstruation (dysmenorrhea)

Painful menstruation (dysmenorrhea) is characterised by excessive uterine contractile activity associated with bleeding. In this disorder, a distinction is made between a primary form (bleeding is painful from the onset of menstruation) and a secondary form (pain occurs in women whose periods have not previously been accompanied by pain). Painful menstruation can be mild, moderate or heavy, depending on the severity of the condition. These include endometriosis, pelvic inflammatory disease, myomas, endometrial polyps, adnexal tumours, adenomyosis, adhesions (e.g. operations in this area). Nicotine addiction, living conditions, changes in lifestyle, stress can all have an impact on the occurrence of the condition. In some women, especially if their periods are painful from the beginning, it is not possible to find the cause of the pain. Particularly worrying is the occurrence of pain in a woman who has not previously had painful periods. It is then necessary to visit a gynaecologist for a check-up.

Infrequent menstruation (oligomenorrhoea)

Another fairly common menstrual disorder is infrequent menstruation (oligomenorrhoea), which is characterised by infrequent bleeding throughout the year. The most common causes are weight-related disorders, chronic diseases, imaginary pregnancy, ovarian failure, polycystic ovary syndrome (PCOS), hyperprolactinaemia.

Is it normal to have irregular periods?

Within 2-3 years of the onset of the first menstrual period (i.e. up to about 15 years of age), menstrual cycles can be irregular in most teenage girls, which is a variant of the norm. The most common reason for this situation is a lack of ovulation or immaturity of the hypothalamic-pituitary-ovarian endocrine system. However, other possible causes should also be ruled out, e.g. endocrine disorders, psychiatric disorders or structural defects within the reproductive system. It should also be borne in mind that the sexual cycles of peri-menopausal women may also be more irregular menstruation. In such cases, deviations from the norm in terms of monthly bleeding do not give cause for concern.

What can cause irregular periods?

If you have an early period, a late period, or missed it altogether, that’s normal if you’ve only been getting your period for three or four months. After that, though, it could mean there’s something else going on.

Hormones are the boss of your reproductive system, controlling everything from ovulation (which is when your ovary releases an egg) to menstruation. They’re in charge of signalling to your body that the egg isn’t fertilised and therefore it’s time to shed the lining of the uterus, which makes up period blood. So, if you have an irregular menstruation, blame it on your hormones.

When your hormones are imbalanced for any reason, it can do some weird things to your period, in terms of both flow and its timing. That’s why irregular periods often happen during your teens or when you first start bleeding; your hormones are usually still getting into the swing of things. Of course, there’s always the possibility that you might be pregnant if you’ve missed a period, in which case your body’s going to hang onto that uterine lining. You could also be menopausal, in which case you’re not ovulating at all. Other than that, there are some fairly common factors that can affect your hormones, in turn leading to irregular menstruations.

What causes irregular periods? Reasons for irregular periods can occur in the course of many diseases and disorders that affect the reproductive system, the endocrine system or the whole body. The balance of a teenager's body processes also depends on her lifestyle or diet.

Read on to learn about a few of the usual suspects - reasons for irregular periods.

Stress and irregular periods

Dealing with a breakup, having a big event on the horizon, or just being absolutely swamped at work? That stress can cause irregular periods. When you’re super stressed, your body sends out hormones to prepare you for flight or fight, slowing down non-essential functions – like those for your reproductive system – throwing off your body’s usual timing in the process. If you’re feeling frazzled 24/7, it could lead to getting your period late.   During severe stress, many changes take place in the body. The sympathetic nervous system is the first to activate; after a while, the hypothalamic-pituitary-adrenal system, the so-called stress axis, becomes active. A number of studies have shown that women with hypothalamic-pituitary-ovarian axis dysfunction have elevated levels of hormones responsible for regulating adrenal function (CRH, ACTH) and cortisol.

Stress and irregular periods cause menstrual disorders can last up to six months. If, after this time, cognitive behavioural therapy does not work, treatment has to be initiated and hormone therapy with estroprogestogens has to be used.

Learn more about how stress can cause irregular periods here.

Changes in your weight

If you lose or gain a significant amount of weight, it could throw a spanner in your body’s hormone balance (Same goes if you’re restricting your calories) - it’s causes of irregular menstruation. They most often occur after weight loss - losing a lot of weight can lead to a hormonal imbalance in the body. Lack of menstruation often accompanies a weight-loss diet combined with training. Once that’s imbalanced, it can lead to a late or missed period.

Polycystic Ovary Syndrome (PCOS)

PCOS is a health condition where your hormones are imbalanced, which can cause irregular periods. With this disease, the ovaries produce large amounts of androgens, hormones that prevent or delay ovulation. As a result, you don’t ovulate regularly and therefore don’t get your period regularly either. You might have a really heavy period, get irregular menstruations, or frequently experience missed periods. And because it’s a hormonal issue, it can come with some rough symptoms of irregular periods, such as acne, excess hair on the face and body, weight gain, skin tags, and dark patches on the skin.

Thyroid or pituitary disorders

Both hyperthyroidism, hypothyroidism and Hashimoto's disease can cause irregular periods and prolonged cycles. However, if up to now your periods have occurred - despite long cycles - regularly, but have not for some time, this is a clue to seek specialist advice. Proper thyroid function is essential for maintaining reproductive function.


Endometriosis occurs when endometrial tissue grows outside the uterus. The tissue often attaches to the ovaries or fallopian tubes. Monthly bleeding as a result of endometriosis may be irregular or often more heavy with a significant increase in additional discomfort. Symptoms may be constant or cyclical. This means that they worsen before and during the period and then improve.

Pelvic inflammatory disease

Pelvic organ inflammation (PID) is a bacterial infection that affects the female reproductive system. It is usually caused by an untreated sexually transmitted infection (STI) - most commonly gonorrhoea, chlamydia, anaerobic bacteria. The bacteria enter the vagina and spread to the uterus and upper genital tract. Symptoms of PID include profuse vaginal discharge with an unpleasant odour, irregular menstrual bleeding, pelvic pain and fever.

Uncontrolled diabetes

If you have uncontrolled diabetes, you may have irregular menstruations because the interaction between blood sugar levels and hormones can interfere with your menstrual cycle. In order to restore normal menstruation, it is necessary to initiate appropriate treatment and to examine oneself thoroughly. Often diabetes is also accompanied by polycystic ovary syndrome.


Some types of contraception work by manipulating your hormones. When you first start or stop taking hormonal contraception, you might have irregular periods as your body gets used to it, but it’s nothing to worry about. However, the body slowly adapts to the doses of hormones provided and the bleeding becomes regular. Most often, such bleeding with the contraceptive pill is less heavy and less painful than menstruation. It is sometimes the case that more or less intense spotting while taking the contraceptive pill can occur after many months of use.

How are irregular periods diagnosed?

It is always necessary to visit a gynaecologist when periods are too infrequent (less than every 35 days), too frequent (more than 21 days), too heavy (loss of more than 100 ml of blood), too scanty (loss of less than 35 ml). The doctor should find the cause of this and implement possible treatment. He or she should take a thorough history, information about the date of the first menstruation, the gynaecological-obstetric past, chronic diseases and much more. Ideally, a woman who notices any kind of menstrual disorder should keep an accurate diary including the date her bleeding starts and ends and the date of ovulation, if she experiences it.

In the general examination, the doctor assesses the possible presence of nutritional disorders, genital development, the presence of acne, excessive hair and many other factors. The ultrasound examination assesses the structure of the uterus, the thickness of the endometrium, the ovaries, the presence of follicles in the ovary, among others.

Additional tests include tests of hormone levels: oestrogen, progesterone, LH, FSH, prolactin, testosterone, androgens. If all laboratory results are normal in a woman with irregular menstruations, the doctor orders further tests, which are hormonal tests, genetic tests (including evaluation of the karyotype, i.e. evaluation of the woman's chromosomes and genes), hysteroscopy - an examination involving the insertion of a camera into the uterine cavity, or imaging tests of the head (magnetic resonance imaging or computed tomography).

An important component of diagnosis is the assessment of mental status and the presence of chronic diseases.

How to regulate your periods

There’s no way to make your period stick to a schedule outside of certain types of contraceptives, which can take the reins from your hormones so you can plan or even skip your period. Otherwise, if your periods are still irregular after a few months and you’re not pregnant, make an appointment to see your doctor. While most irregular periods regulate themselves, a women’s health specialist like an OB-GYN can diagnose something more serious, such as PCOS. So how to regulate your periods? In addition to seeing your doctor, first and foremost take care of yourself. Rational nutrition, for example, will help regulating menstrual cycle. It is also a good idea to try active recreation to reduce the tension resulting from daily stresses. It is also worth thinking about vitamin support.

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