What makes a period "irregular"?
A period is considered irregular when cycle length varies significantly from month to month — typically defined as variation of more than 7–9 days. If your cycle is sometimes 24 days and sometimes 38 days, that's irregular. If your cycle is consistently 35 days every month, that's actually regular — just longer than average.
Research suggests that up to a third of people who menstruate experience cycle-to-cycle variation greater than 7 days at some point in their reproductive lives. So while irregular periods can sometimes signal an underlying condition worth investigating, they're also extremely common as a temporary response to life circumstances.
The key distinction is between occasional variation (one off cycle, then back to normal) and persistent irregularity (cycles that consistently vary by large amounts). The former is normal. The latter is worth paying attention to — not because it's necessarily dangerous, but because it's often a signal from your body about something that deserves understanding.
The most common causes of irregular periods
Many conditions and circumstances can cause irregular cycles. Here are the most frequently encountered:
- PCOS (polycystic ovary syndrome): The single most common hormonal cause of irregular periods. PCOS disrupts the hormonal balance needed for regular ovulation, resulting in cycles that can range from very long to absent entirely. It affects around 1 in 10 women of reproductive age.
- Thyroid disorders: Both an overactive thyroid (hyperthyroidism) and underactive thyroid (hypothyroidism) affect menstrual regularity. Thyroid hormones are closely involved in regulating the reproductive system. A simple blood test (TSH) checks for this.
- Hyperprolactinemia: Elevated prolactin — the hormone associated with breastfeeding — can suppress ovulation and cause irregular or absent periods even when not breastfeeding.
- Chronic stress: Sustained high cortisol from ongoing emotional or physical stress suppresses the hormonal axis regulating ovulation, causing persistent cycle irregularity.
- Significant weight changes or extreme exercise: Both very low body fat and rapid weight change affect oestrogen production and can disrupt cycle regularity.
- Perimenopause: From the early 40s (sometimes earlier), fluctuating hormone levels cause cycles to become irregular — sometimes shorter, sometimes longer, sometimes skipped.
- Endometriosis: While primarily associated with painful periods, endometriosis can also cause cycle irregularity in some cases.
PCOS and irregular cycles — what to know
If your cycles have always been irregular or became irregular in your teens or twenties, PCOS is worth considering. It's one of the most underdiagnosed conditions in women — partly because symptoms vary so much between individuals and partly because irregular periods are so often dismissed.
PCOS is diagnosed using a combination of criteria: irregular cycles, evidence of elevated androgens (on blood test or physical examination), and the appearance of multiple follicles on the ovaries on ultrasound. You don't need all three — two of the three is sufficient for diagnosis under the Rotterdam criteria.
This calculator was built partly with PCOS in mind. The range-based approach — entering your shortest and longest cycle rather than a single average — gives you a much more useful prediction than any fixed-date calculator. If you're trying to conceive with PCOS, your fertility window is also a range rather than a single day, which is reflected in the ovulation window estimate this calculator provides.
A note from Kanika: I have PCOS. My cycles have ranged from 26 days to 58 days. Every single standard period app I tried was useless — it kept predicting dates that were wildly wrong because it assumed I had a consistent cycle. The whole reason this calculator exists is because I needed something that worked for cycles like mine. And like yours.
Dive Deeper: The Biology of Irregularity
Did you know you can ovulate without bleeding, or have highly sporadic fertile windows? Understand the science of silent ovulation.
Read: Can You Ovulate Without a Period? →How to track an irregular cycle effectively
The single most useful habit for irregular cycles is consistently logging the first day of each period. That's the data this calculator needs, and it's the foundation of understanding your own pattern. Even three months of data gives you a working shortest and longest cycle to enter here.
Beyond date tracking, physical signs can tell you a lot about where you are in your cycle when date prediction isn't reliable:
- Cervical mucus changes: Egg-white consistency signals you're approaching ovulation, regardless of what day of the cycle you're on
- LH test strips: Detect the hormone surge 12–36 hours before ovulation — useful when ovulation timing is unpredictable
- Basal body temperature (BBT): A small rise after ovulation confirms it has occurred, even if timing was later than expected
Combining date-based range estimates (like this calculator provides) with physical signs gives you the best possible picture of your cycle even when it's irregular. No single method is perfect, but the combination is far more reliable than any one tool alone.
When should you speak to your doctor?
Irregular cycles don't always require medical attention, but there are situations where a conversation with your GP is genuinely worthwhile:
- Your cycles vary by more than 20 days consistently, or you have fewer than 8 periods per year
- Irregular periods are a new development — they used to be regular and have changed
- You have accompanying symptoms: unexplained weight gain, excessive hair growth or thinning, severe acne, pelvic pain, or significant fatigue
- You're trying to conceive and the unpredictability of your cycle is making it difficult
- Your periods are very heavy or very painful in addition to being irregular
A GP can order the key blood tests — hormone panel, thyroid function, blood glucose and insulin if PCOS is suspected — and can refer you to a specialist if needed. Early diagnosis of conditions like PCOS or thyroid disorders makes management significantly easier and more effective.