What Causes Irregular Periods?
Most people with periods have some sense of their rough cycle rhythm- roughly how long between periods, roughly how long the bleeding lasts. When that rhythm stops being reliable, it's natural to want to know why. A period that shows up ten days early, a cycle that stretches past seven weeks, or a gap that's impossible to explain can all leave you wondering whether something is wrong or whether this is just how bodies are sometimes.
The honest answer is that irregular periods have a wide range of causes- from things that resolve on their own, like stress or a recent illness, to conditions that benefit from clinical assessment, like PCOS or thyroid changes. Most irregular cycles don't signal anything serious. But patterns that persist across multiple months, or that come with other symptoms, tend to mean more than a single out-of-place cycle.
This guide covers what clinicians mean by irregular, the most common causes, and how to think about when irregular cycles are worth investigating rather than waiting out.
What counts as an irregular period
Clinicians generally consider a typical cycle to fall between 21 and 35 days, measured from the first day of one period to the first day of the next. Some variation within that range- a few days either side from month to month- is normal and doesn't indicate a problem.
Irregular periods, in the clinical sense, tend to mean one or more of the following:
- Cycles that consistently fall outside the 21-35 day range
- Cycle lengths that vary significantly from month to month- for example, 24 days one cycle, 41 the next, 29 after that
- Periods that stop for three months or more without pregnancy as the explanation
- Bleeding patterns that have recently changed substantially from what was previously normal for you
One cycle that falls outside your usual pattern is common and usually doesn't warrant concern. When the unpredictability itself becomes the pattern- most months something is different- that's the point at which understanding the cause becomes more useful.
Make sense of your cycle pattern
Cycle changes are surprisingly hard to assess from memory alone. People tend to remember the most recent cycle clearly and recall others only vaguely, which makes it difficult to tell whether irregularity is getting better, getting worse, or staying roughly the same. The Menstrual Cycle Irregularity Checker asks structured questions about your recent cycles- lengths, timing changes, associated symptoms- and organises the pattern into something clearer before you try to explain it to a clinician.
Common causes of irregular periods
Stress and major life changes
Psychological and physical stress both affect the hormonal signals that regulate ovulation. The hypothalamus, which coordinates the hormonal cycle, is sensitive to stress hormones- when cortisol levels are persistently elevated, the signalling that drives ovulation can be suppressed or delayed, which lengthens cycles. This is also why significant illness, surgery, or a period of extreme physical strain can temporarily disrupt cycle regularity. The disruption typically resolves once the underlying stressor does, though that may take a few months.
Weight changes and energy availability
The hormonal machinery that drives ovulation requires a certain level of energy availability. Significant weight loss, very low body fat, or the kind of calorie restriction associated with disordered eating can all suppress ovulation, causing cycles to lengthen or stop. Rapid weight gain can also affect hormone levels in ways that disrupt cycle regularity. The effect isn't linear and doesn't have a precise threshold- it depends on individual physiology and how quickly changes occur.
Exercise intensity
High-volume or high-intensity exercise, particularly when combined with inadequate calorie intake to match the energy expenditure, can suppress the hormones needed for regular ovulation. This is sometimes called relative energy deficiency in sport (RED-S). Cycles may lengthen or stop entirely. The effect tends to be dose-dependent- recreational exercise rarely causes this, but training at an elite or near-elite level, particularly in weight-sensitive sports, raises the risk considerably.
Stopping hormonal contraception
Hormonal contraceptives- the pill, the patch, the injection, the implant- work partly by suppressing the natural hormonal cycle. When you stop using them, it takes time for the body to re-establish its own rhythm. For most people, cycles return within a few months. For others, particularly after long-term use of certain methods, the adjustment period is longer. Irregular cycles in the months after stopping contraception are normal; persistent irregularity beyond six months is worth mentioning to a clinician.
Polycystic ovary syndrome (PCOS)
PCOS is one of the most common causes of persistently irregular cycles. It involves a hormonal imbalance that affects how the ovaries develop and release eggs- ovulation may be infrequent or absent, producing long cycles or gaps between periods. PCOS often comes with other features: acne that clusters around the jaw, increased facial or body hair, and sometimes difficulty maintaining weight. It's diagnosed through a combination of clinical history, blood tests, and ultrasound, and it's manageable with both medical and lifestyle approaches.
Thyroid conditions
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can disrupt the menstrual cycle. Hypothyroidism tends to produce heavier, more frequent, or irregular periods, while hyperthyroidism can cause lighter, less frequent, or absent periods. Thyroid conditions usually come with other symptoms- fatigue, temperature sensitivity, weight changes, hair changes- but cycles can be the first thing that shifts. Thyroid function is one of the first things checked in a standard fertility workup.
Elevated prolactin
Prolactin is a hormone involved in milk production, but it's also present in people who aren't breastfeeding. Elevated prolactin levels- from a benign pituitary growth called a prolactinoma, certain medications, or other causes- can suppress ovulation and cause cycles to become irregular or stop. It's detectable through a blood test and treatable once identified.
Perimenopause
For people in their 40s, increasing cycle variability can reflect the early stages of the hormonal shift toward menopause. Cycles may become shorter, then longer, then unpredictable, with changes in bleeding volume and duration. This transition can begin earlier than many people expect- sometimes in the late 30s. If you're under 45 and cycles are becoming irregular alongside other symptoms like hot flashes or sleep disruption, it's worth mentioning to a clinician, as premature ovarian insufficiency is a separate consideration.
Structural conditions
Fibroids, polyps, and adenomyosis can all alter bleeding patterns. Fibroids and polyps are non-cancerous growths in or on the uterus that can change how heavy or long a period is; adenomyosis involves uterine lining tissue growing into the muscle wall and typically produces heavy, painful, and sometimes irregular periods. These conditions are diagnosed through clinical examination and imaging rather than through symptoms alone.
Other causes
Certain medications- including antipsychotics, some antidepressants, and chemotherapy agents- can affect cycle regularity. Significant chronic illness can also disrupt the hormonal cycle. If cycles changed around the time a new medication started, that's worth raising with the prescribing clinician.
Reasons for Abnormal Periods - What Makes a Period Irregular
Abnormal periods include cycles that are too short (under 21 days), too long (over 35 days), too heavy, too light, or absent altogether. The reasons for abnormal periods fall into two broad categories: hormonal disruption and structural causes. Hormonal causes - including stress, thyroid dysfunction, PCOS, and elevated prolactin - affect the signals that trigger ovulation and menstruation. Structural causes - including fibroids, polyps, and adenomyosis - affect the uterine lining directly.
Why Have My Periods Become Irregular - Sudden Changes vs Long-Term Patterns
If your periods have recently become irregular after previously being regular, the most common reasons are stress, significant weight change, stopping hormonal contraception, or the early stages of perimenopause. A sudden change in cycle regularity that persists for more than two or three cycles is worth investigating, especially if it comes with other symptoms like pain, heavy bleeding, or unexplained fatigue.
What this could mean over time
A cycle that's a week later than usual tells you something happened- illness, a stressful month, a disrupted routine. A year of cycles that are consistently unpredictable tells a clinician something quite different.
Event: Your last period was 18 days later than expected after a particularly demanding work period.
Pattern: Looking at the past nine months, cycle lengths have ranged from 27 to 52 days. There's no clear trigger for the longer cycles- some fell during high-stress periods, others didn't. Three of the last nine cycles were over 40 days.
Insight: When high variability persists across most cycles without a clear external trigger, and especially when some cycles are substantially longer than typical, that's a pattern worth investigating clinically- not because it confirms any particular cause, but because it changes what questions a clinician should be asking.