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Irregular Periods: 9 Common Causes & When to See a Doctor
An occasional off month is normal. A cycle that's regularly unpredictable is your body asking for attention — here's what's usually behind it.
Most people's cycles aren't metronome-perfect, and a little variation is completely normal. But when periods regularly arrive early, late, or not at all, there's usually a reason worth understanding. Here's what "irregular" actually means and the nine most common causes.
What counts as an irregular period?
A typical cycle runs 21 to 35 days. Your periods are generally considered irregular if the length regularly falls outside that range, swings widely from month to month (say, 24 days then 38 then 29), or if you frequently skip periods altogether. One unusual month after a stressful stretch isn't a red flag; a persistent pattern is what's worth looking into.
9 common causes of irregular periods
- Stress. Sustained stress raises cortisol, which can suppress the hormones that drive ovulation — delaying or skipping periods.
- Significant weight change. Rapid weight loss or gain, and very low body fat, can disrupt the hormonal signals behind a regular cycle.
- Intense exercise. High training loads, especially combined with under-eating, can push periods to become light or absent.
- PCOS (polycystic ovary syndrome). One of the most common hormonal causes of irregular or missed periods, often alongside acne, excess hair growth, or difficulty losing weight.
- Thyroid disorders. Both an under- and over-active thyroid can change cycle length and flow.
- Perimenopause. In the years before menopause (often the 40s), shifting hormones make cycles longer, shorter, and less predictable.
- Hormonal contraception. Starting, stopping, or switching birth control commonly changes bleeding patterns for a few months.
- The first years after your first period. It's normal for cycles to be irregular for a couple of years as the system matures, and again while breastfeeding.
- Certain medications and conditions. Some medications, as well as conditions like diabetes or elevated prolactin, can affect your cycle.
Why tracking helps
If your cycle is irregular, a tracking record turns vague "it's all over the place" into specific data — your real range, how flow changes, and which months differ. That record is genuinely useful in a doctor's office. Here's how to track your period step by step.
When to see a doctor
Book an appointment with a clinician if you notice any of the following:
- Your periods suddenly change pattern after being regular.
- You miss three or more periods in a row and aren't pregnant.
- Bleeding is very heavy (soaking through protection hourly) or lasts longer than seven days.
- You bleed between periods or after sex.
- You have severe pain, or your cycles are consistently shorter than 21 or longer than 35 days.
None of these mean something is necessarily wrong — but they point to causes that are very often treatable once identified, from thyroid issues to PCOS. Understanding where ovulation fits in can also help; see our guides to the menstrual cycle phases and ovulation signs.
Frequently asked questions
What is considered an irregular period?
Cycle lengths that regularly fall outside 21–35 days, vary widely month to month, or frequent missed periods. Occasional variation is normal.
What causes irregular periods?
Most often stress, weight change, intense exercise, PCOS, thyroid disorders, perimenopause, hormonal contraception, and natural life stages.
When should I see a doctor?
If periods suddenly change, you miss three or more in a row without pregnancy, bleeding is very heavy or long, you bleed between periods, or you have severe pain.