If your cycle never seems to arrive on schedule — or skips months entirely — you're not alone. Here's what's behind an irregular menstrual cycle and what you can do about it.
Most people learn that a "normal" cycle is 28 days, but that figure is more myth than medicine. Healthy cycles can range from 21 to 35 days and still be perfectly regular. What matters more than any single number is consistency: does your cycle arrive on roughly the same schedule each month?
An irregular menstrual cycle is one that falls outside the 21–35 day window, or one that varies by more than seven days from cycle to cycle — for example, arriving at day 25 one month and day 38 the next. Occasional variation is normal, especially around times of stress. But persistent irregularity is your body's way of flagging that something may need attention.
Irregular periods can stem from lifestyle factors, hormonal shifts, or underlying health conditions. Here are the ten most common culprits.
Chronic physical or emotional stress raises cortisol levels, which can suppress the hormonal signals that trigger ovulation. When ovulation is delayed or skipped, your period follows suit — arriving late, early, or not at all. Even a single stressful event, like an exam period, a bereavement, or a major life change, can push a cycle off-track by a week or more. Once the stressor resolves and cortisol levels normalise, cycles typically return to their usual pattern.
PCOS is one of the most common hormonal conditions in women of reproductive age, affecting roughly 1 in 10. It disrupts the balance of oestrogen, progesterone, and testosterone, causing irregular or absent ovulation. People with PCOS may have very infrequent periods (oligomenorrhoea) or none at all (amenorrhoea). Other signs include excess facial or body hair, acne, and difficulty managing weight. PCOS is manageable but needs a formal diagnosis from a healthcare provider.
The thyroid gland produces hormones that regulate metabolism — and those hormones interact closely with your reproductive hormones. An underactive thyroid (hypothyroidism) often causes heavier, more frequent periods, while an overactive thyroid (hyperthyroidism) can lead to lighter, less frequent cycles or missed periods altogether. Thyroid disorders are among the most under-diagnosed causes of irregular periods and are easily ruled in or out with a simple blood test.
Both significant weight loss and rapid weight gain can disrupt the hormones that regulate ovulation. Body fat plays an active role in oestrogen production, so when fat stores drop too low — through restrictive eating, an eating disorder, or rapid dieting — oestrogen falls and periods can stop. On the other side, excess body fat can increase oestrogen and insulin levels, disrupting the cycle in the opposite direction. Gradual, sustainable changes to body weight tend to have less impact on cycle regularity.
Intense athletic training — particularly when combined with low calorie intake — can suppress the reproductive hormones needed for ovulation. This is known as relative energy deficiency in sport (RED-S) and is common among long-distance runners, gymnasts, and competitive athletes. The resulting condition, sometimes called athletic amenorrhoea, can have long-term consequences for bone density if left unaddressed. Reducing training volume or increasing caloric intake often restores regular cycles within a few months.
In the years leading up to menopause — typically from the mid-40s onward, though sometimes earlier — oestrogen and progesterone levels begin fluctuating unpredictably. This perimenopausal transition can cause cycles to become shorter, longer, heavier, lighter, or to skip months entirely. Hot flushes, sleep disturbances, and mood changes often accompany the irregular periods. Perimenopause is a natural phase, but tracking symptoms and discussing them with a doctor can help identify the right support.
After stopping the pill, a hormonal IUD, implant, or injection, it can take several months — sometimes up to a year — for your natural cycle to re-establish itself. This is particularly common after the contraceptive injection (Depo-Provera), which can suppress periods for many months after the last dose. The body simply needs time to resume its own hormone production. If your cycle hasn't returned within 12 months of stopping contraception, it's worth checking in with a healthcare provider.
Prolactin — the hormone responsible for breast milk production — also suppresses ovulation. This is why many breastfeeding parents experience absent or irregular periods, especially during the first several months of exclusive breastfeeding. As feeds become less frequent or as solid foods are introduced, prolactin levels drop and the menstrual cycle usually returns. The timing varies widely from person to person; some see their period return within weeks of birth while others don't until weaning is complete.
Fibroids are non-cancerous growths within or around the wall of the uterus; polyps are smaller overgrowths of the uterine lining. Both can interfere with the regularity and flow of periods. They may cause heavier or longer bleeding, spotting between periods, or cycles that come more or less frequently than usual. Fibroids and polyps are very common — particularly in women in their 30s and 40s — and are typically diagnosed via ultrasound. Many don't require treatment unless symptoms are bothersome.
Not every irregular cycle points to a problem. Cycles can shift naturally at different life stages — in the first few years after menarche (first period), the body is still calibrating its hormonal rhythms, so irregular cycles are the norm. Similarly, illness, travel across time zones, a disrupted sleep pattern, or even a change in work schedule can nudge a cycle off-track temporarily. If the irregularity is mild, occasional, and not accompanied by other symptoms, it may simply be normal variation.
Spotting patterns in an irregular cycle is much easier when you have your data. WomensPal logs your cycle length, symptoms, and flow every month — all in one place.
Start tracking free →Occasional variation in your cycle is rarely cause for alarm. But there are certain situations where speaking to a healthcare provider sooner rather than later is important:
A doctor will typically start with a detailed history and blood tests to check hormone levels (FSH, LH, oestradiol, prolactin, thyroid hormones, and sometimes androgens). An ultrasound scan may follow to look at the ovaries and uterus. Most causes of irregular periods are very treatable once identified.
Management depends entirely on the underlying cause. For lifestyle-driven irregularity, the steps are relatively straightforward:
For condition-driven irregularity (PCOS, thyroid disorders, fibroids, and so on), treatment will be tailored by your healthcare provider and may include hormonal contraception to regulate cycles, thyroid medication, medication to manage PCOS symptoms, or in some cases a minor procedure to address fibroids or polyps.
Yes, they can — but "irregular periods" doesn't automatically mean "infertility." Irregular cycles often reflect irregular ovulation, and ovulation is what's required for conception. Conditions like PCOS can make conception harder but are also among the most treatable causes of ovulatory infertility. If you're trying to conceive and your periods are irregular, speaking to a doctor sooner rather than later gives you more options and more time.
Absolutely. Stress is one of the most common reasons for a temporarily delayed period. When the body perceives stress, it prioritises survival over reproduction — cortisol suppresses the hormone signals needed to trigger ovulation, which pushes back the timing of your period. Once the stressor passes and your nervous system settles, your cycle usually returns to its usual pattern within one or two cycles. Chronic, unrelenting stress can, over time, cause more persistent irregularity.
It varies. Most people see their natural cycle return within one to three months of stopping a combined oral contraceptive. For the contraceptive injection, the return to regular periods can take six to twelve months or even longer. If your periods haven't returned — or haven't become regular — within twelve months of stopping any hormonal contraception, a visit to your GP is worthwhile to rule out any underlying issues that the contraception may have been masking.
If excess body weight is contributing to the irregularity (as it can in PCOS or insulin resistance), then moderate, sustainable weight loss may help restore more regular ovulation. However, losing weight too quickly or dropping below a healthy body weight can have the opposite effect, suppressing periods further. The goal is healthy, steady progress rather than rapid restriction. A GP or dietitian can help you find the right approach for your individual situation.
Tracking an irregular cycle is slightly different from tracking a regular one — you can't rely on a predicted date. Instead, focus on recording the first day of each period as it arrives, your flow level, duration, and any associated symptoms like pain or mood changes. Over time (ideally across at least three cycles), you'll start to identify your personal range. A period tracking app like WomensPal makes this easy, storing your data and highlighting patterns without requiring a regular schedule.