Irregular periods are common, affecting an estimated 14% to 25% of women of childbearing age. The causes range from everyday factors like stress and weight changes to hormonal conditions that need treatment. Understanding what counts as “irregular” in the first place, and what might be driving it, can help you figure out whether your body is doing something perfectly normal or signaling that something needs attention.
What Counts as an Irregular Period
A normal adult menstrual cycle falls between 21 and 34 days, measured from the first day of one period to the first day of the next. Bleeding typically lasts seven days or fewer. If your cycles consistently fall outside that window, come more frequently than every 24 days, or stretch beyond 38 days apart, that’s considered irregular by medical standards.
Some variation from month to month is expected. A cycle that’s 28 days one month and 31 the next isn’t cause for concern. What matters more is the overall pattern: cycles that swing wildly in length, periods that disappear for months, or bleeding that’s dramatically heavier or lighter than your usual baseline. If you’re in your first few years of menstruating, a wider range is normal. About 90% of adolescent cycles fall between 21 and 45 days, and it can take three years after your first period for cycles to settle into a tighter adult range.
Stress and Your Cycle
When your body is under sustained stress, whether from work, relationships, illness, or major life changes, it produces more of the stress hormone cortisol. Cortisol interferes with the brain signals that trigger ovulation. Specifically, it reduces the frequency of hormonal pulses from your brain that tell your ovaries to release an egg. Without that signal firing on schedule, ovulation gets delayed or skipped entirely, which pushes your period later or causes you to miss it altogether.
This isn’t a subtle effect. Significant physical or psychological stress can shut down the reproductive hormonal chain enough to stop periods for months. Research on women undergoing intense physical and psychological stress (such as military basic training) found that this type of hormonal suppression is remarkably common. The good news is that stress-related irregularity usually resolves once the stressor eases or you develop better ways to manage it.
Weight, Exercise, and Energy Balance
Your body needs a minimum amount of available energy to support a menstrual cycle. When you’re burning more calories than you’re taking in, whether through intense exercise, restrictive eating, or both, your fat cells produce less of a hormone called leptin. Leptin acts as an energy gauge for your brain. When levels drop too low, your brain interprets this as a signal that your body can’t support a pregnancy, and it dials down the hormones that drive ovulation.
This condition, called hypothalamic amenorrhea, leads to missed periods and, over time, bone loss and other hormonal disruptions affecting your thyroid and adrenal function. It’s especially common in endurance athletes, dancers, and anyone restricting food intake significantly. Research published in PNAS demonstrated that restoring leptin levels in women with this condition brought back menstruation and corrected the cascading hormonal problems. In practical terms, that means the fix is usually nutritional: eating enough to match your activity level.
On the other end of the spectrum, carrying significantly more body weight can also disrupt your cycle. Excess fat tissue produces estrogen, and too much estrogen without the counterbalancing effect of progesterone (which your body only makes after ovulation) can cause irregular, heavy, or prolonged bleeding.
Thyroid Problems
Your thyroid gland controls your metabolism, but it also has a direct line to your reproductive hormones. Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can disrupt the hormonal balance needed for ovulation. An underactive thyroid has an additional effect: it can cause your body to overproduce prolactin, the hormone responsible for breast milk production. Elevated prolactin suppresses ovulation even when you’re not pregnant or breastfeeding, leading to missed or infrequent periods.
Thyroid disorders are one of the more treatable causes of irregular periods. Once thyroid hormone levels are brought back into range with medication, cycles often normalize on their own.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common reasons for persistently irregular periods in women of reproductive age. It involves higher-than-normal levels of androgens (sometimes called “male hormones,” though all women produce them in small amounts). These elevated androgens interfere with the development and release of eggs from the ovaries, leading to infrequent or absent periods. Many women with PCOS also notice acne, excess hair growth on the face or body, or difficulty losing weight.
If your periods have been irregular since adolescence and you recognize some of those symptoms, PCOS is worth exploring with a healthcare provider. It’s diagnosed based on a combination of symptoms, blood work, and sometimes ultrasound, and it responds to a range of treatments depending on whether your primary concern is cycle regularity, fertility, or symptom management.
Hormonal Contraceptives
If you’ve recently started or switched birth control, irregular bleeding is one of the most common side effects. Modern contraceptive pills contain lower doses of estrogen than older formulations, and those lower doses sometimes aren’t enough to keep the uterine lining stable. The lining thins and breaks down unpredictably, causing spotting or breakthrough bleeding between periods.
Progestin-only methods, including certain pills, hormonal IUDs, and injections, are particularly likely to cause irregular bleeding patterns because they work by thinning the uterine lining over time. This can mean frequent spotting for the first several months, lighter periods, or periods that stop altogether. For most women, bleeding patterns settle after three to six months on a new method. Stopping hormonal birth control can also temporarily throw off your cycle as your body recalibrates its own hormone production.
Perimenopause
If you’re in your 40s and your previously predictable cycle has started behaving unpredictably, perimenopause is a likely explanation. This transitional phase before menopause can begin as early as your mid-30s, though it most commonly starts in your mid-40s. During perimenopause, estrogen and progesterone levels fluctuate erratically rather than following their usual monthly rhythm. Your cycles may get shorter, then longer, then shorter again. Periods might be heavier some months and barely there the next.
Hormone testing during perimenopause is notoriously unreliable because levels swing so dramatically from day to day. A high FSH (follicle-stimulating hormone) level can suggest you’re approaching menopause, but a single test result doesn’t tell the full story. Most providers diagnose perimenopause based on your age, symptoms, and the pattern of changes you describe rather than a blood test.
Other Medical Causes
Several other conditions can cause irregular periods. Uterine fibroids and polyps are noncancerous growths that can lead to heavy or prolonged bleeding. Elevated prolactin levels from causes other than thyroid disease (such as certain medications or a small benign pituitary growth) can suppress ovulation. Uncontrolled diabetes and significant changes in body weight in either direction can also play a role.
Signs That Need Medical Evaluation
Some patterns of irregular bleeding warrant a closer look. Cycles that consistently come fewer than 24 days apart or more than 38 days apart fall outside the normal range. Bleeding that soaks through a pad or tampon in an hour or less, periods lasting longer than seven days, or bleeding between periods or after sex are all worth bringing up with a provider.
To find the cause, your provider will likely order blood tests checking levels of thyroid-stimulating hormone, prolactin, testosterone, cortisol, and follicle-stimulating hormone. Together, these give a picture of where in the hormonal chain something might be off. For women 45 and older with abnormal bleeding, or younger women with certain risk factors like obesity or a family history of endometrial cancer, an endometrial biopsy may be recommended to rule out changes in the uterine lining.
Tracking your cycles for two to three months before your appointment, noting cycle length, flow heaviness, and any spotting, gives your provider much more useful information than a vague description of “irregular periods.” Several free apps make this easy, or a simple calendar notation works just as well.