Irregular periods have dozens of possible causes, ranging from hormonal imbalances and stress to thyroid problems and the natural transition toward menopause. A normal menstrual cycle falls between 24 and 38 days. If the gap between your shortest and longest cycles varies by more than 7 to 9 days (depending on your age), your periods are considered irregular.
What Counts as Irregular
The international standard for regularity depends on how much your cycle length bounces around, not just whether it hits a specific number. For people aged 26 to 41, cycles that vary by more than 7 days are classified as irregular. For those aged 18 to 25 or 42 to 45, the threshold is a bit wider: variation of more than 9 days. So if one month your cycle is 26 days and the next it’s 37, that 11-day swing would qualify as irregular regardless of your age group.
Periods that come fewer than 8 times a year, stop for 3 or more months, or suddenly become much heavier or lighter than usual also fall outside the normal range.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal causes of irregular periods. It develops when the body produces higher than normal levels of androgens (sometimes called “male hormones,” though everyone makes them). That excess can interfere with ovulation, the monthly release of an egg that triggers a period roughly two weeks later. Without regular ovulation, periods become unpredictable or disappear altogether.
People with PCOS often have cycles longer than 35 days apart, or fewer than 8 periods a year. Other signs include acne, excess facial or body hair, and thinning hair on the scalp. A diagnosis typically requires at least two of three features: elevated androgen levels or their visible effects, irregular ovulation, and a characteristic appearance of the ovaries on ultrasound. PCOS is treatable, and managing it often restores more regular cycles.
Thyroid Problems
Your thyroid gland and your reproductive hormones are controlled by neighboring systems in the brain, and they influence each other closely. When thyroid hormone levels drift too high or too low, the ripple effects reach your menstrual cycle.
An underactive thyroid (hypothyroidism) is more commonly linked to infrequent periods and heavier bleeding. An overactive thyroid (hyperthyroidism) tends to cause lighter periods. The mechanism is complex: thyroid hormones alter how your body handles estrogen, change levels of a protein that carries sex hormones through the blood, and can even shift androgen production. Even subtle differences in thyroid function appear to affect cycle length, primarily by changing how long the first half of your cycle lasts.
Thyroid disorders are diagnosed with a simple blood test and are highly treatable. Once thyroid levels are brought back to normal, menstrual regularity usually follows.
Stress and Energy Deficits
Chronic stress disrupts periods through a specific chain of events. When cortisol (the body’s primary stress hormone) stays elevated, it suppresses the brain signal that kicks off your entire menstrual cycle. Cortisol reduces the frequency of pulses of gonadotropin-releasing hormone, the master signal that tells the pituitary gland to produce the hormones needed for ovulation. It also makes the pituitary less responsive to that signal and can delay the hormonal surge that triggers ovulation. The result: late, skipped, or absent periods.
Physical stress works the same way. When your body doesn’t get enough fuel relative to how much energy you’re burning, it starts shutting down non-essential systems, and reproduction is one of the first to go. Research shows that when energy availability drops below about 30 calories per kilogram of lean body mass per day, the probability of menstrual disturbance exceeds 50 percent. There isn’t a hard cutoff where periods suddenly stop. Instead, the risk climbs steadily as the energy gap widens. This is why irregular or missing periods are common among people who are dieting aggressively, exercising heavily without eating enough, or both. The condition is called functional hypothalamic amenorrhea, and it’s reversible with adequate nutrition and reduced training intensity.
Perimenopause
Perimenopause is the transition leading up to menopause, and it’s one of the most common reasons periods become irregular in your 40s. Most people notice changes sometime in that decade, though some see shifts as early as their mid-30s or as late as their early 50s. During this phase, estrogen levels fluctuate unpredictably rather than following their usual monthly pattern. Cycles may get shorter, then longer. You might skip a month, then have two heavy periods in a row.
This is a normal biological transition, not a disorder. It typically lasts 4 to 8 years before periods stop entirely. The irregularity itself doesn’t require treatment, though the symptoms that come with it (hot flashes, sleep disruption, mood changes) often do.
High Prolactin Levels
Prolactin is the hormone responsible for milk production, but it also has a powerful effect on the menstrual cycle. When prolactin levels are too high outside of pregnancy or breastfeeding, a condition called hyperprolactinemia, it suppresses the brain’s release of reproductive hormones and can shut down ovulation entirely.
The earliest sign is often a shortened second half of the cycle (the luteal phase), which may show up before periods become noticeably irregular. As prolactin levels climb further, periods may become infrequent or stop altogether. Common causes include small benign growths on the pituitary gland, certain medications (especially some psychiatric drugs and anti-nausea medications), and sometimes no identifiable cause at all. Treatment depends on the underlying reason but often restores normal cycles.
Hormonal Contraceptives
If you’ve recently started a progestin-only method of birth control, irregular bleeding is extremely common, not a sign that something is wrong. More than 50 percent of people taking progestin-only pills experience changes in their menstrual pattern, and roughly 40 percent have irregular bleeding during the first three to six months of use. Hormonal IUDs and implants can cause similar irregularity, especially early on.
This typically improves with time. After the first six months, many users find their bleeding becomes lighter and more predictable, and some stop bleeding altogether. Combination pills (containing both estrogen and progestin) can also cause spotting in the first few months but tend to regulate cycles rather than disrupt them.
Fibroids and Polyps
Structural growths in the uterus can make periods heavier, longer, or less predictable. Fibroids are noncancerous muscle growths in the uterine wall. They most commonly cause heavy periods, pelvic pressure, and a feeling of fullness. Polyps are smaller growths that develop on the uterine lining and are more associated with spotting between periods and irregular cycle timing.
Both are common, especially after age 30, and neither is cancerous in the vast majority of cases. They’re usually found during an ultrasound or a procedure where a small camera is inserted into the uterus. Treatment depends on symptoms: small, symptom-free growths are often just monitored, while larger or problematic ones can be removed.
Other Contributing Factors
Several additional conditions can throw off your cycle. Uncontrolled diabetes affects hormone regulation. Celiac disease and other conditions that impair nutrient absorption can mimic an energy deficit. Significant weight gain or loss in either direction alters estrogen levels, since fat tissue is actively involved in estrogen production. Even travel across time zones or a major shift in your sleep schedule can temporarily delay ovulation.
In adolescents, irregular periods are common for the first two to three years after menstruation begins as the hormonal feedback system matures. This is generally normal and resolves on its own. However, adolescents who soak through a pad in under two hours, have periods lasting longer than 7 days, or have a family history of bleeding disorders should be evaluated, as an underlying clotting problem may be the cause.
Signs That Need Medical Attention
Occasional irregularity is common and often harmless. But certain patterns warrant a visit to your healthcare provider: periods that stop for three or more months when you’re not pregnant, cycles consistently shorter than 24 days or longer than 38 days, bleeding between periods, or bleeding that has become dramatically heavier than your baseline. Soaking through a pad or tampon every hour for several hours is considered acute bleeding and needs prompt attention.
For anyone 45 or older, new irregular bleeding should be evaluated because the risk of endometrial cancer increases with age. For younger people, persistent irregularity paired with signs like excess hair growth, significant weight changes, or milky discharge from the nipples points toward specific hormonal causes that are straightforward to test for with blood work and imaging.