Irregular periods have a long list of possible causes, from stress and thyroid problems to structural changes in the uterus. A “regular” menstrual cycle falls between 21 and 35 days, with bleeding lasting 2 to 7 days. If your cycles fall outside that window, vary by more than 7 to 9 days from month to month, or disappear for 3 to 6 months at a time, something is shifting your hormonal balance or affecting your reproductive system.
What Counts as Irregular
Many people assume their period needs to arrive every 28 days to be normal, but that number is just an average. Cycles anywhere from 21 to 35 days are considered typical, and some natural variation from month to month is expected. What matters more than hitting a specific number is consistency. If your cycle is 32 days one month and 34 the next, that’s fine. If it swings from 25 days to 45 days, that’s the kind of irregularity worth investigating.
ACOG considers bleeding abnormal if your cycles are shorter than 21 days, longer than 35 days, or vary by more than 7 to 9 days cycle to cycle. Going 3 to 6 months without a period (when you’re not pregnant) also qualifies. Spotting between periods or after sex, bleeding that lasts longer than 7 days, or soaking through a pad or tampon every hour are additional signs that something beyond normal variation is going on.
Stress and Its Effect on Ovulation
Stress is one of the most common and most overlooked causes of irregular periods. When your body is under sustained physical or emotional stress, it ramps up production of cortisol, the primary stress hormone. Cortisol directly suppresses the brain signals that trigger ovulation. Specifically, it inhibits the release of a hormone called GnRH from the hypothalamus, which is the starting signal for your entire menstrual cycle. Without that signal, your ovaries may not release an egg on schedule, or at all.
This isn’t limited to major life crises. Chronic work pressure, sleep deprivation, intense exercise routines, or significant calorie restriction can all raise cortisol enough to delay or skip ovulation. The result is a late period, a skipped period, or cycles that bounce around unpredictably. Once the stressor resolves, cycles typically return to normal within a few months, though it can take longer if the stress was prolonged.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal conditions affecting people of reproductive age, and irregular periods are its hallmark symptom. The condition involves a combination of elevated levels of androgens (hormones like testosterone that are typically higher in men), problems with ovulation, and sometimes cysts on the ovaries visible on ultrasound. A diagnosis requires at least two of those three features under the widely used Rotterdam criteria.
In PCOS, the hormonal imbalance interferes with the normal monthly process of maturing and releasing an egg. Without regular ovulation, periods become unpredictable. You might go weeks or months between cycles, or have very long cycles of 40, 60, or even 90 days. Other signs that point toward PCOS include acne, excess facial or body hair, thinning hair on the scalp, and difficulty losing weight. If your irregular periods come alongside any of these symptoms, PCOS is worth discussing with your doctor. It’s diagnosable with blood work and an ultrasound, and it responds well to treatment.
Thyroid Problems
Your thyroid gland, a small butterfly-shaped organ at the front of your neck, produces hormones that influence nearly every system in your body, including your reproductive system. Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can disrupt your cycle, but they do it through different mechanisms.
Hypothyroidism is the more clearly understood culprit. When thyroid hormone levels drop too low, your body compensates by increasing production of a brain hormone called TRH. TRH has a side effect: it stimulates the release of prolactin, a hormone best known for triggering breast milk production. Elevated prolactin suppresses the GnRH signal that kicks off ovulation, effectively putting the brakes on your cycle. The result can be infrequent periods, heavier than usual bleeding, or periods that stop altogether. Studies show that key reproductive hormones like LH and FSH are suppressed in hypothyroid women and bounce back once thyroid levels are corrected.
Hyperthyroidism can also cause menstrual irregularity and missed ovulation, though the exact mechanism is less well mapped. The good news is that for both conditions, treating the thyroid problem usually restores regular cycles.
Perimenopause
If you’re in your 40s and your previously predictable period has started acting unpredictable, perimenopause is the most likely explanation. This transitional phase begins about 8 to 10 years before menopause, typically starting in the mid-40s, though it can begin as early as the mid-30s. During perimenopause, your ovaries gradually produce less estrogen, and hormone levels rise and fall erratically rather than following their usual monthly pattern.
The hallmark of perimenopause is changing cycle length. You might have a 24-day cycle followed by a 40-day cycle, or skip a month entirely. Periods may become heavier or lighter. Hot flashes, sleep disruption, and mood changes often show up alongside the cycle changes. Your doctor can check FSH levels (a hormone that rises as the ovaries slow down), but a single test isn’t always reliable during this phase because hormones fluctuate so much. The pattern of your symptoms over time is often more telling than any one blood draw.
Uterine Polyps and Fibroids
Not all irregular periods stem from hormonal problems. Structural growths inside the uterus can physically disrupt bleeding patterns. Uterine polyps are small, usually noncancerous growths that develop on the lining of the uterus. They’re a common cause of both irregular timing and unusually heavy flow. Many people with polyps notice unpredictable spotting between periods or bleeding that doesn’t follow any recognizable schedule.
Fibroids, which are muscular growths in the uterine wall, tend to cause heavier and longer periods rather than changes in cycle timing, though they can contribute to irregularity too. Both conditions are typically diagnosed with an ultrasound and are treatable. Polyps can often be removed in a minor procedure, and fibroids have a range of treatment options depending on their size and location.
Other Common Causes
Several other factors can shift your cycle without signaling a major medical condition. Significant weight changes in either direction affect estrogen production, since fat tissue plays a role in hormone metabolism. Gaining or losing more than about 10% of your body weight in a short period can throw off ovulation. Starting, stopping, or switching hormonal birth control is another frequent cause. It can take your body several months to re-establish a natural rhythm after stopping the pill, an IUD, or an injection.
Breastfeeding suppresses ovulation through the same prolactin pathway involved in thyroid-related irregularity. Travel, illness, and shift work can also disrupt the signals your brain sends to your ovaries, since your reproductive system is sensitive to changes in sleep and circadian rhythm.
When Irregular Periods Need Attention
Some patterns call for a prompt conversation with a healthcare provider. Going more than 3 months without a period (when you’re not pregnant or breastfeeding) suggests something is consistently preventing ovulation. Cycles that are consistently shorter than 21 days or longer than 35 days deserve evaluation. Spotting between periods or after sex can indicate polyps, infections, or cervical changes that should be checked.
Seek emergency care if you are soaking through pads or tampons every hour for more than 2 hours in a row, especially if you also feel lightheaded, dizzy, or short of breath. That level of bleeding can lead to dangerous blood loss and needs immediate treatment. Any bleeding that occurs after menopause, even light spotting, should always be evaluated.