Irregular Periods: Causes, When It Is Normal and When to See a Doctor

Many women notice that their periods do not always arrive like clockwork. Variation of a few days is completely normal. But persistently irregular, very heavy, very light or missed periods may signal a hormone imbalance or another health issue. This guide explains what is normal and what is not, the causes, and when to get help.

What is a normal cycle?

• Cycle length: counted from the first day of one period to the first day of the next. Normal for adults is about 24 to 38 days, and the length should not change by more than 7-9 days from cycle to cycle.
• Bleeding length: up to about 8 days.
• Amount of bleeding: up to about 80 mL in total. Soaking through a pad every hour for several hours, passing large clots, or bleeding that interferes with daily life is heavy.
• Teenagers often have irregular cycles for the first 2-3 years after periods begin.
• Perimenopause, the years before menopause (usually from the 40s), commonly brings irregular cycles.

What counts as irregular?

• Cycles shorter than 21-24 days or longer than 35-38 days
• Missing three or more periods in a row (amenorrhoea) without pregnancy
• Cycles that vary widely in length
• Bleeding between periods or after sex
• Very heavy or very light bleeding
• Periods that stop after being regular, or never start by age 15-16

Common causes

• Pregnancy (always rule this out first) and breastfeeding
• Polycystic ovary syndrome (PCOS): a very common cause, with acne, excess hair and weight gain. See PCOS symptoms and treatment
• Thyroid disorders: over- or under-active thyroid
• High prolactin (pituitary hormone), sometimes from medicines or a small pituitary growth
• Stress, illness, travel, or shift work
• Weight changes: significant loss, eating disorders, or obesity
• Intense exercise, low body fat
• Hormonal contraception: starting, changing or stopping the pill, implant, injections or hormonal IUD
• Perimenopause and premature ovarian insufficiency (periods stop before 40)
• Uterine problems: fibroids, polyps, endometriosis, adenomyosis, or scarring
• Medicines: some antidepressants, antipsychotics, steroids and chemotherapy
• Chronic diseases: diabetes, coeliac disease, liver or kidney disease
• Reproductive tract infections

Tests your doctor may suggest

• Pregnancy test
• Blood tests: thyroid hormone (TSH), prolactin, FSH, LH, testosterone, estradiol, blood count and iron, blood sugar
• Pelvic ultrasound to check the ovaries, womb and lining
• Further tests if needed: endometrial biopsy for abnormal bleeding in older women, hysteroscopy or MRI

Treatment depends on the cause

• Lifestyle: maintain a healthy weight, eat regularly, sleep well, manage stress and avoid extreme exercise or crash diets. Use our BMI calculator as a guide.
• Hormonal treatment: the combined pill, progestogen tablets or a hormonal IUD to regulate cycles and protect the womb lining.
• Thyroid or prolactin treatment if abnormal.
• PCOS management: weight loss, diet, metformin, hormonal medicines, and fertility treatment if needed.
• Treat fibroids, polyps or endometriosis with medicines or surgery as appropriate.
• Iron supplements if bleeding has caused anaemia.
• Fertility support when trying to conceive.

Irregular periods and pregnancy

Ovulation is unpredictable with irregular cycles, so calendar-based methods are unreliable. Using an ovulation calculator is only an estimate. If you are trying to conceive, see your doctor after 6-12 months, or earlier with irregular cycles. Read how to get pregnant faster.

Can irregular periods affect health?

Yes, if ovulation is infrequent, the womb lining can build up and, over many years, increase the risk of endometrial hyperplasia and cancer. Low oestrogen from missing periods can weaken bones. Heavy bleeding can cause iron-deficiency anaemia. Treating the cause protects your long-term health.

Self-care tips

• Keep a period diary or app, noting start dates, flow, pain and spotting.
• Eat balanced meals with iron-rich foods such as dhal, green leafy vegetables and fish.
• Aim for regular exercise, but avoid overtraining.
• Limit smoking, alcohol and sugary drinks.
• Do not take medicines or herbal products to "bring on" a period without medical advice, especially if you could be pregnant.

When to see a doctor

See a doctor if:

• You have missed three periods in a row or have not had a period for more than 90 days and are not pregnant
• Cycles are shorter than 21 days or longer than 38 days regularly
• Periods have not started by age 15-16
• You bleed between periods, after sex or after menopause
• Bleeding is very heavy, with dizziness, tiredness or large clots
• You have severe pelvic pain, fever or foul-smelling discharge
• You have milky nipple discharge, rapid weight change, excessive hair growth or hot flushes under age 40
• You are trying to conceive and cycles are irregular

References

• FIGO: normal and abnormal uterine bleeding system
• American College of Obstetricians and Gynecologists: menstruation in girls and adolescents
• NICE guideline NG88: heavy menstrual bleeding
• International evidence-based guideline on PCOS, 2023