Irregular periods: what counts, common causes and what helps

What counts as irregular?

Hardly anyone has a perfect 28-day cycle. In a study of more than 600,000 cycles tracked in an app, the average was about 29 days, and most people's cycles varied by a few days from month to month. So some variation is completely normal.

Teenagers are an exception: cycles from 21 to 45 days are common in the first few years after periods start, while ovulation settles into a rhythm. Perimenopause, usually in your 40s, brings irregularity back.

Why cycles become irregular

Most irregularity comes down to ovulation. When ovulation is delayed or skipped, the cycle stretches, and the period that eventually comes can be heavier or lighter than usual. The things that most often disturb ovulation:

• Life stage: the first years after periods start, after pregnancy, while breastfeeding and in perimenopause
• Stress and big life changes, from exams to grief
• Energy balance: significant weight loss or gain, under-eating, or a big increase in training
• Hormonal contraception, starting, switching or stopping it
• PCOS (polycystic ovary syndrome, which some health services now call PMOS), one of the most common causes, often with acne, extra hair growth or weight changes
• Thyroid conditions, both under- and overactive
• High prolactin, sometimes from medicines or a small, benign pituitary growth
• Conditions affecting the uterus, such as polyps or fibroids, which cause irregular bleeding rather than irregular cycles

What helps

Track your real pattern

The single most useful thing is a record. Log the first day of every period, how long you bleed and anything unusual for at least three to six months. That shows whether you're mildly variable or genuinely irregular, and gives a doctor something concrete. Our cycle length calculator turns a few dates into your average and range.

Support ovulation with the basics

• Eat enough, regularly. Under-fueling is a common, under-recognized cause, especially in people who exercise a lot.
• Keep sleep steady and protect it during stressful weeks.
• If you have PCOS, even a modest change in weight (where appropriate), regular activity and balanced meals can help cycles become more regular.

Treatment depends on the cause

A doctor may arrange a pregnancy test, blood tests for thyroid, prolactin and other hormones, and sometimes an ultrasound. Treatment then targets the cause: thyroid medicine, managing PCOS, or hormonal options that regulate bleeding. If you're trying to conceive, irregular cycles are worth raising early, since ovulation tracking is harder.

Irregular doesn't mean unpredictable forever

Even irregular cycles often have patterns: signs like egg-white discharge, ovulation tests or a wrist temperature rise tell you ovulation has happened and your period is roughly two weeks away.

Irregular periods and mood

Not knowing when your period will come is draining. You can't plan, PMS seems to come out of nowhere, and every late cycle brings a pregnancy question. Many people with irregular cycles find their mood still follows their own rhythm: the low, irritable days still come in the run-up to bleeding, just not on a fixed calendar date. Logging mood next to your cycle helps you see that link. Read more in irregular cycles tracking.