What cause irregular periods ?

Your period rarely shows up when you expect, it might start a week early, last a few days longer or shorter than usual, be slightly heavier or lighter than usual.

Irregular periods are common at different stages of a woman’s reproductive life. In teenagers, up to 80% may experience irregular cycles during the first one to two years after their periods begin, as their bodies are still establishing a regular hormonal rhythm. Among adult women, around 20 to 30% experience some degree of irregular menstruation at any given time, including changes in cycle length, flow, or missed periods. Irregular cycles also become more common during perimenopause, when fluctuating hormone levels cause the ovaries to produce less oestrogen and progesterone.

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.

The exact time between bleeding can be anywhere from 21 to 35 days – it just depends on your body. 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.

My periods have vanished – why?

If your periods haven’t started by age 15 it is known as primary amenorrhea. Secondary amenorrhea is when menstrual bleeding stops for three months or more in a woman who previously had regular periods.

There are lots of reasons why periods might not start or might stop suddenly, and in many cases, it’s nothing dangerous. The most common reason for a missed period is pregnancy, but other factors can play a role too. Some people have anatomical differences, like an imperforate hymen or missing reproductive organs, while others may have hormonal conditions such as high prolactin levels or an underactive thyroid. Certain health conditions, including autoimmune disorders, can affect the ovaries and lead to periods stopping earlier than expected. Lifestyle factors like high levels of stress, very intense exercise, certain medications or changes in eating habits can all also affect your cycle.

Within the first few years after the first period, menstrual cycles are often irregular.

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.

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 perimenopausal, where ovulation becomes irregular, or menopausal where there is no ovulation at all and the periods have stopped.

What causes irregular periods?

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

Stress

Dealing with a breakup, having a big event on the horizon, or just being absolutely swamped at work? Stress, be that emotional or physical, can disrupt yor menstrual cycle and can cause irregular periods. Stress activates the body’s stress-response systems, including the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. Increased levels of cortisol and corticotropin-releasing hormones can suppress the hypothalamic-pituitary-ovarian (HPO) axis, which regulates ovulation. This may lead to delayed periods, missed periods, or irregular bleeding.

Learn more about how stress can cause irregular periods here.

Changes in your weight

Gaining or losing a significant amount of weight, or restricting calories could throw a spanner in your body’s hormone production and ovulation. Low body weight, excessive exercise, or inadequate calorie intake can lead to reduced oestrogen levels and irregular or absent periods. But weight gain can also contribute to menstrual irregularities by affecting insulin resistance and hormonal balance. Restoring a stable, healthy weight often helps normalise the menstrual cycle.

Polycystic Ovary Syndrome (PCOS)

PCOS is a common hormonal condition which can cause irregular or absent periods. Here higher levels of androgen hormones and insulin resistance can interfere with normal ovarian function. Other symptoms can include acne, excess hair growth, difficulty managing weight, skin tags, and darkened skin patches. PCOS is a long-term condition, but symptoms can be effectively managed through lifestyle changes, medical treatment, and hormonal regulation.

Thyroid or pituitary disorders

Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause irregular periods and prolonged cycles. Pituitary gland conditions, including prolactin disorders (hyperprolactinaemia), can also affect ovulation and menstrual timing.

Endometriosis

Endometriosis occurs when tissue similar to the lining of the uterus grows outside the uterus, often on the ovaries, fallopian tubes or bowel. Endometriosis causes symptoms such as pelvic pain and heavy, painful periods, bleeding patterns can also become irregular. Symptoms may be persistent or cyclical, worsening before and during menstruation. Diagnosis and management may include pain relief, hormonal treatment, or surgical options depending on severity and symptoms.

Pelvic inflammatory disease

Pelvic organ inflammation (PID) is a bacterial infection that affects the reproductive organs. It is usually caused by an untreated sexually transmitted infection (STI) such as gonorrhoea or, chlamydia. Symptoms may include abnormal vaginal discharge, pelvic or abdominal pain, fever, pain during intercourse, and irregular bleeding. It is important that PID is treated with antibiotics.

Uncontrolled diabetes

Poorly controlled diabetes can affect the menstrual cycle, influencing the HPO axis and ovulation. Treatment aims to stabilise blood sugar levels and manage any associated conditions such as PCOS, which often coexists with insulin resistance. Improved diabetic control usually leads to more regular cycles.

Contraception

Hormonal contraception such as combined pills, progestogen-only pills, patches, injections, implants, and hormonal IUDs can all alter your menstrual cycle. Irregular bleeding, spotting, lighter periods, or absence of periods are common, especially in the first few months while the body adjusts.

Generally these changes are safe and expected, for example some women choose to use the combined oral contraceptive pill to have lighter or less frequent bleeds. Over time, bleeding often becomes lighter and more predictable, though some methods (particularly progestogen-only methods) may continue to cause irregular bleeding for longer. Stopping hormonal contraception may also lead to temporary changes in your cycle as your body gets used to the change.

How are irregular periods diagnosed?

You tell us! It can be useful to track your periods, perhaps by using an app, and if your periods come either less than 21 days apart or more than 35 days please see your doctor. Unless you are on a form of hormonal contraception that stops bleeding it is important that you have a period three or four times a year to prevent the lining of the uterus becoming too thick. Your doctor may request investigations such as hormone blood tests.

How to regulate your periods

There isn’t a guaranteed way to make your period run like clockwork unless you use certain hormonal contraceptives, which can help make your cycle more predictable or even let you safely delay a bleed. If your periods are still all over the place after a few months and you know you’re not pregnant, it’s a good idea to check in with your doctor.

Looking for ways to help your cycle settle down? Start with taking care of yourself. Eating regular, balanced meals and keeping your body well-nourished can really support your hormones. Staying active and finding little ways to unwind can help reduce stress, which is a common culprit behind irregular cycles. And if you’re thinking about vitamins or supplements, it’s best to chat with a healthcare professional first to make sure you’re taking what your body actually needs and won’t cause harm or interact with other medications

This is generalised advice and isn't personalised nor to replace the advice of your doctor and other health care professionals.