An irregular period means the number of days between your cycles changes significantly from month to month, or your cycles fall outside the typical 21-to-35-day range for adults. This is one of the most common gynecological concerns, and the causes range from temporary and harmless (like stress) to conditions that benefit from treatment (like thyroid disorders or PCOS). Understanding what’s behind your irregular cycles starts with knowing what “regular” actually means and which factors are most likely disrupting yours.
What Counts as Irregular
A menstrual cycle is measured from the first day of bleeding in one period to the first day of bleeding in the next. For adults, a normal cycle falls between 21 and 35 days. For adolescents, the window is wider: 21 to 45 days is considered typical during the first few years after a first period, because the hormonal system is still maturing. By the third year after menarche, 60 to 80 percent of cycles settle into the 21-to-34-day adult range.
Your period is considered irregular if it consistently comes more often than every 21 days or less often than every 35 days (45 for teens), if you go more than 90 days without a period even once, or if your bleeding lasts longer than 7 days. Occasional variation of a few days is normal. What matters is a pattern of unpredictability or cycles that consistently fall outside these ranges.
Stress and Your Cycle
Stress is one of the most common and underestimated reasons periods become irregular. When your body is under sustained physical or emotional stress, it produces higher levels of cortisol. Cortisol directly interferes with the hormonal signals that trigger ovulation. Specifically, it slows down the brain’s release of the hormone that orchestrates your entire cycle, reducing its pulse frequency by as much as 45 to 70 percent in research models. That delay can push ovulation back by days or skip it entirely for a cycle, which shows up as a late, missed, or unusually light period.
This isn’t just about major life crises. Chronic work pressure, sleep deprivation, illness, or even travel can raise cortisol enough to throw off your cycle. The effect is usually temporary: once the stressor resolves, cycles tend to normalize within one to three months.
PCOS: The Most Common Hormonal Cause
Polycystic ovary syndrome affects roughly 1 in 10 women of reproductive age and is the leading hormonal cause of irregular periods. PCOS disrupts ovulation, so cycles become long, unpredictable, or absent altogether. You might go 40, 60, or 90-plus days between periods, or have fewer than 8 cycles in a year.
PCOS is diagnosed when at least two of three features are present: irregular cycles (signaling disrupted ovulation), elevated levels of androgens (male-type hormones that can cause excess facial or body hair, acne, or thinning scalp hair), and a characteristic appearance of the ovaries on ultrasound or a blood marker called AMH. Not everyone with PCOS has all three, and the condition looks different from person to person. Some people have obvious symptoms like stubborn acne and hair growth along the jawline or chest. Others have irregular cycles as their only noticeable sign.
If your cycles are consistently longer than 35 days and you notice any signs of excess androgen, PCOS is worth investigating with your doctor. It’s manageable with treatment, but it doesn’t resolve on its own.
Thyroid Problems
Both an underactive and overactive thyroid can disrupt your period, but hypothyroidism (underactive) is the more common culprit. The connection is surprisingly indirect. When thyroid hormone levels drop, your brain produces more of a signaling hormone called TRH to try to stimulate the thyroid. TRH also happens to stimulate prolactin, a hormone normally associated with breastfeeding. Elevated prolactin then suppresses the hormonal cascade that drives ovulation.
About 20 percent of people with even mild hypothyroidism have prolactin levels high enough to interfere with their cycle. The result can be longer cycles, heavier or lighter bleeding, or missed periods entirely. A simple blood test for thyroid function can identify this, and treating the thyroid issue typically restores regular cycles.
Body Weight and Exercise
Your body needs a minimum threshold of energy availability and body fat to sustain a menstrual cycle. When you fall below that threshold, whether from restrictive eating, intense exercise, or a combination, the brain dials down reproductive hormones to conserve energy. This condition, called functional hypothalamic amenorrhea, is especially common in athletes, dancers, and anyone experiencing significant caloric restriction.
The exact body fat percentage that triggers this varies from person to person. Research has not pinpointed a single number. What’s clear is that the combination of low energy intake, high exercise intensity, and elevated cortisol from physical stress all contribute. Some women lose their period at a body fat percentage that another woman tolerates fine, because individual hormonal sensitivity differs. On the other end of the spectrum, carrying significantly excess weight can also cause irregularity by increasing estrogen production from fat tissue, which disrupts the normal hormonal rhythm and can prevent ovulation.
Hormonal Contraceptives
If you recently started, stopped, or switched a hormonal contraceptive, irregular bleeding is extremely common and usually temporary. Up to 30 percent of women experience abnormal bleeding in the first month on combination birth control pills. For progestin-only methods, the numbers are even higher: more than half of progestin-only pill users have changes to their bleeding pattern, and up to 70 to 80 percent of women using injections or implants experience unpredictable bleeding during the first year.
This typically improves after three to six months as your body adjusts. Stopping hormonal contraception can also cause irregular cycles for several months while your natural hormonal rhythm re-establishes itself.
Perimenopause
If you’re in your 40s (or sometimes late 30s) and your previously clockwork cycle has become unpredictable, perimenopause is a likely explanation. This transition phase before menopause can last several years, and it’s driven by fluctuating and gradually declining estrogen and progesterone levels. Cycles may get shorter, then longer, then skip entirely. Flow might be heavier some months and barely there the next.
Most women notice perimenopausal changes in their 40s, though some experience them as early as their mid-30s or as late as their 50s. There’s no single blood test that definitively confirms perimenopause. The pattern of increasingly irregular cycles in the right age range is usually enough to identify it.
Structural Causes: Fibroids and Polyps
Not all irregular bleeding is hormonal. Growths in or on the uterus can physically alter your bleeding pattern. Fibroids, which are noncancerous muscular growths in the uterine wall, tend to cause heavy periods, pelvic pressure, and severe menstrual cramps. Polyps, which are smaller growths on the uterine lining, are more associated with spotting between periods and unpredictable cycle timing.
Both are common, especially in women over 30. They’re typically detected through ultrasound. Fibroids and polyps don’t always require treatment, but when they cause disruptive symptoms, several options exist ranging from medication to minor procedures.
Signs That Need Prompt Attention
Some patterns of irregular bleeding signal something that shouldn’t wait. Soaking through a pad or tampon every hour for several consecutive hours suggests abnormally heavy bleeding that can lead to anemia. Feeling persistently tired, weak, or short of breath alongside irregular periods may mean you’re already anemic from blood loss. Spotting between periods, even if light, is worth mentioning to a provider because it can indicate polyps, infection, or other issues that benefit from early evaluation.
Any bleeding after you’ve fully transitioned to menopause (12 consecutive months without a period) is never considered normal and should always be evaluated. The same applies to any bleeding during pregnancy.