Irregular periods causes: 8 common reasons and when to see a doctor

A period that is a few days late, or one that arrives at a different time every month, worries many women. In fact irregular periods are common and are often linked to stress, weight change or hormonal shifts. Sometimes, though, a disrupted menstrual cycle is a sign of a condition that needs a doctor's attention — such as polycystic ovary syndrome or a thyroid problem. This article explains what a normal cycle looks like, the main causes of a late period, what tests a doctor may order and which symptoms should not wait.

What does a normal menstrual cycle look like?

A cycle is counted from the first day of one period to the first day of the next. According to the NHS (the UK's National Health Service):

• the average cycle is 28 days, but anything from 21 to 35 days is common;
• a period lasts 2 to 7 days, usually about 5, with the heaviest bleeding in the first two days;
• typical blood loss per period is about 20–90 ml (roughly 1–5 tablespoons);
• periods usually start around age 12, and most girls have regular periods by 16–18;
• menopause usually happens between the mid-40s and mid-50s.

Every woman has her own "normal". A difference of a day or two from month to month is usually nothing to worry about. Pay attention if your cycle length changes suddenly or new symptoms appear.

What counts as a disrupted cycle?

Doctors distinguish several types of changes:

Change	What it looks like	What to do
Irregular cycle	Gap between periods shorter than 21 or longer than 35 days	Keep a calendar and show your doctor
Late or missed period	No period when expected	Pregnancy test first; three missed in a row — see a doctor
Long period	More than 7 days	See a doctor
Heavy bleeding	Changing a pad every 1–2 hours, large clots	See a doctor
Bleeding between periods	On other days of the cycle or after sex	Always get checked

Causes of a late period: rule out pregnancy first

If you are sexually active and your period is late, the first step is to rule out pregnancy — even if you used contraception. A pharmacy test is simple and quick; if the result is unclear, a doctor may order a blood test (hCG).

If the test is negative and your period still has not come, the cause lies elsewhere. Below are the most common ones.

8 main causes of irregular periods

1. Stress and anxiety

Strong emotional strain, exams, pressure at work or family problems affect the hormonal control centre in the brain. Ovulation can be delayed, and the period with it. Usually the cycle returns to normal once stress eases.

2. Sudden weight change

Losing a lot of weight quickly, strict diets or, on the other hand, excess weight can disrupt cycle regularity. Fat tissue takes part in hormone metabolism, so both very low and very high weight can affect the cycle.

3. Polycystic ovary syndrome (PCOS)

PCOS is one of the most common causes of irregular or infrequent periods. It often comes with weight gain, extra hair on the face and body, oily skin or acne. Some sources, including the NHS, have started using a new name for it — PMOS. More details are on our PCOS page.

4. Thyroid problems

Both too little (hypothyroidism) and too much (hyperthyroidism) thyroid hormone can change the cycle. There may also be tiredness, weight change, sensitivity to cold or heat, or a fast heartbeat.

5. High prolactin

Prolactin is a hormone linked to milk production. If it is raised outside breastfeeding, periods may be delayed or stop, and sometimes there is fluid from the breasts. A doctor finds the cause with blood tests and, if needed, further examination.

6. Perimenopause

In the years before menopause, usually between 45 and 55, cycles may become longer or shorter and periods less frequent — a natural process. But very heavy bleeding or bleeding between periods still needs checking in this phase.

7. Hormonal contraception and breastfeeding

Hormonal pills, injections and the hormonal coil can change your periods — often an expected effect. During breastfeeding, periods may also stay away for a long time. If in doubt, talk to the doctor who prescribed the method.

8. Excessive exercise and chronic illness

Very intense training, especially combined with eating too little, can disrupt the cycle. Diabetes, heart disease and other chronic conditions can affect it too.

PCOS and insulin resistance: how are they linked to the cycle?

In PCOS, eggs often do not mature regularly, so periods are late or absent for long stretches. In many women this goes together with insulin resistance: cells respond less well to insulin and the body makes more of it, which also affects hormonal balance.

That is why with PCOS doctors look not only at the cycle but at metabolism as a whole: weight, blood sugar, cholesterol, blood pressure. Lifestyle — balanced eating, regular activity, enough sleep — comes first. Whether hormonal or other medicines are needed is decided by a gynaecologist or endocrinologist.

What tests will a doctor order?

The list depends on your age, symptoms and plans (for example, pregnancy). A doctor usually starts with:

• a conversation: cycle calendar, weight changes, stress, medicines, contraception;
• a pregnancy test or blood hCG;
• a full blood count — especially with heavy bleeding, to check for anaemia;
• thyroid hormones (TSH and others);
• prolactin and sex hormones on specific days of the cycle;
• blood sugar and, if needed, markers of insulin resistance;
• a pelvic ultrasound scan.

Your doctor will tell you when and on which day of the cycle to take the tests — some hormones change with the cycle day, so tests done "just in case" are often misread.

What can you do at home?

1. Keep a calendar. Note the start and end of each period, how heavy it is and any pain. A phone app works too. This is very valuable information for your doctor.
2. Look after sleep and rest. Around 7–8 hours of sleep and ways to reduce stress (walks, breathing exercises) help.
3. Avoid strict diets. Include protein, vegetables, legumes and whole grains; cut down on sweets and white flour.
4. Move in moderation. Regular moderate activity is good; exhausting overtraining is not.
5. Do not take hormonal medicines on your own. Pills or herbs that "bring on a period" taken without advice can hide the real cause.

When to see a doctor

Book a routine appointment with a gynaecologist if:

• you have missed three periods in a row;
• periods have not started by age 15;
• your cycle has become irregular or periods last more than 7 days;
• late periods come with weight gain, tiredness, facial hair growth or skin changes;
• you bleed between periods or after sex;
• your periods are very painful;
• irregular cycles make it hard to get pregnant.

See a doctor without delay or call emergency services if you have:

• very heavy bleeding: changing a pad every 1–2 hours, clots larger than about 2.5 cm, blood soaking through clothes or bedding;
• bleeding with dizziness, feeling faint or severe weakness;
• a positive pregnancy test with severe lower abdominal pain (especially on one side) or bleeding;
• any bleeding after menopause (more than a year without periods) — even if light or only once;
• lower abdominal pain with fever and unusual discharge.

How Nutva Inositol may help

Inositol is one of the substances studied in PCOS, but it is not a remedy that "restores" the cycle. The 2023 international PCOS guideline regards inositol as an approach still at the research stage: some results are promising, but high-quality evidence is lacking. So decide whether to take it together with your doctor. Read more in what inositol is, and about the form of folate in the product in folate vs methylfolate.

Nutva Inositol is a biologically active supplement (BFQ) made by «NUTVA PHARM» MChJ (Uzbekistan). Each capsule contains 1000 mg of myo-inositol (vitamin B8) and 200 mcg of 5-methylfolate (vitamin B9) (2 capsules: 2000 mg and 400 mcg), 60 capsules per pack. It supports normal function of the nervous system and cellular processes.

• Permit number: №1594430 (Ministry of Health of Uzbekistan).
• The product is not a medicine.
• During pregnancy, breastfeeding or if you take diabetes medicines — only after consulting your doctor. It does not replace PCOS treatment prescribed by a gynaecologist or endocrinologist.
• How to take it — follow the instructions on the pack and your doctor's advice.

Supplements do not replace a balanced diet, sleep, activity and prescribed medicines.

Frequently asked questions

How many days late is a period before I should worry?

A difference of a day or two is often normal. If your period is late, take a pregnancy test first. If your cycle is regularly longer than 35 days or you miss three periods in a row, see a doctor.

Is pregnancy the only reason for a late period?

No. Stress, sudden weight change, PCOS, thyroid problems, high prolactin, hormonal contraception, breastfeeding and perimenopause can all be causes. A doctor finds the exact cause through examination.

Can stress delay a period?

Yes, strong stress and anxiety can shift the cycle. It usually settles once stress eases, but if delays keep happening or other symptoms appear, get checked.

Which doctor should I see for irregular periods?

Start with a gynaecologist. If needed, they will refer you to an endocrinologist — for example, for thyroid, prolactin or insulin resistance issues.

Does inositol regulate periods?

Inositol is not a medicine for cycle problems. It is being studied in PCOS, but the evidence is still limited. Make any decision about the supplement with your doctor.

What should I do if my periods are very heavy?

If you change a pad every 1–2 hours, pass large clots or your period lasts longer than 7 days, see a doctor. If you feel dizzy or faint, seek help urgently.