Irregular periods have a wide range of causes, from everyday stress and weight changes to hormonal conditions like PCOS and thyroid disorders. A normal menstrual cycle falls between 21 and 35 days, with bleeding lasting 2 to 7 days. If your cycles consistently fall outside that window, or you have fewer than 8 cycles per year, something is likely disrupting the chain of hormonal signals that drives ovulation.
Understanding which category your irregularity falls into can help you figure out whether it’s a temporary disruption or something worth investigating further.
How Your Brain Controls Your Cycle
Your period isn’t just a uterine event. It’s orchestrated by a signaling chain that starts in a small region of the brain called the hypothalamus, travels to the pituitary gland, and ends at the ovaries. This system (sometimes called the HPO axis) releases a carefully timed sequence of hormones that triggers an egg to mature, prompts ovulation, and prepares the uterine lining for a potential pregnancy. When the signal is disrupted at any point along the chain, ovulation can be delayed, skipped entirely, or happen inconsistently, and your period follows suit.
Most causes of irregular periods trace back to something interfering with this signaling pathway, whether it’s a hormone imbalance, a medication, or a structural problem in the uterus itself.
PCOS: The Most Common Hormonal Cause
Polycystic ovary syndrome is one of the most frequent reasons for irregular or missing periods. It’s a complex condition with reproductive, metabolic, and psychological features, and it doesn’t look the same in every person. The core issue is that the ovaries produce excess androgens (hormones typically associated with male development), which can prevent eggs from maturing and releasing on schedule.
Insulin resistance plays a role in many cases, driving the ovaries to produce even more androgens. The result is infrequent ovulation, which shows up as long gaps between periods, very few periods per year, or periods that stop altogether. Excess hair growth on the face or body is another hallmark and, on its own, is considered a strong predictor of PCOS in adults.
One important detail: ovulation problems can still occur even when your cycle looks regular on the calendar. A cycle that arrives every 30 days isn’t guaranteed to include ovulation. This matters most if you’re trying to conceive, since a period without prior ovulation means no egg was released that month.
Thyroid Disorders
Your thyroid gland has a surprisingly direct connection to your menstrual cycle. Thyroid hormones help regulate the brain signals that kick off ovulation each month. When the thyroid underperforms (hypothyroidism), those signals weaken. Levels of key reproductive hormones like LH and FSH drop, and the hormonal cascade that triggers ovulation slows down or stalls.
Hypothyroidism also raises prolactin, a hormone best known for triggering milk production after childbirth. Outside of breastfeeding, elevated prolactin suppresses the brain’s ovulation signals, compounding the problem. The typical pattern with an underactive thyroid is heavier, more frequent periods, though some people experience the opposite.
An overactive thyroid (hyperthyroidism) tends to produce the reverse pattern: lighter, less frequent periods, or periods that disappear for months. Both conditions are treatable, and menstrual regularity often improves once thyroid levels normalize. Studies have shown that LH and FSH levels rise back to normal in hypothyroid women after treatment restores proper thyroid function.
Stress, Sleep, and Energy Balance
Your reproductive system is designed to shut down when your body perceives a threat to survival, and it defines “threat” broadly. Emotional stress, sleep deprivation, and not eating enough calories all register as danger signals that suppress the hormonal chain controlling ovulation.
Research on women undergoing US Army basic combat training found that the combination of elevated stress hormones, fewer hours of sleep, and nutritional strain led to widespread suppression of reproductive function. The effects ranged from shortened luteal phases (the second half of your cycle) to skipped ovulation and missed periods entirely. Some of these disruptions weren’t even detectable through simple period tracking alone.
You don’t need to be in military training for this to apply. Chronic work stress, grief, major life transitions, or prolonged anxiety can produce the same hormonal suppression. The mechanism is straightforward: stress hormones like cortisol interfere with the brain’s release of reproductive hormones, and ovulation gets deprioritized.
Undereating and Low Body Fat
When your body isn’t getting enough fuel relative to your activity level, reproductive function is one of the first systems to scale back. This is especially common in athletes and people with restrictive eating patterns. Low calorie intake, high energy demands, and low body fat percentage all trigger hormonal changes that can make periods irregular or stop them completely.
The threshold isn’t a single number that applies to everyone, but guidelines suggest athletes may need at least 30 to 45 calories per kilogram of fat-free body mass per day to maintain normal hormonal function. Falling below that consistently can suppress the cycle. This isn’t limited to elite athletes. Anyone in a sustained caloric deficit, including from unintentional undereating during busy or stressful periods of life, can experience the same effect.
Structural Problems in the Uterus
Not all irregular bleeding stems from hormonal disruption. Fibroids and polyps are noncancerous growths that can develop inside the uterus or on the cervix. Fibroids form from the muscle tissue of the uterus, while polyps grow from the uterine lining. Both can cause irregular or heavy menstrual bleeding that may not follow a predictable pattern.
Adenomyosis is a related condition where the tissue that normally lines the uterus grows into the muscular wall instead. It typically causes progressively heavier periods and worsening menstrual pain over time. These structural issues affect the bleeding itself rather than the hormonal timing of your cycle, which is why the irregularity often shows up as unpredictable spotting, prolonged bleeding, or unusually heavy flow rather than missed periods.
Medications That Disrupt Your Cycle
Several classes of medication can cause periods to become irregular or stop entirely. The most common mechanism is raising prolactin levels, which suppresses the brain’s ovulation signals. Medications known to do this include antipsychotics, certain antidepressants (including SSRIs and tricyclics), opioid pain medications, and some drugs used for digestive conditions and high blood pressure.
Hormonal contraceptives are another obvious cause. Progestin-based methods like hormonal IUDs, implants, and certain birth control pills can lighten periods significantly or eliminate them, which is often intentional. After stopping hormonal birth control, it can take several months for your natural cycle to re-establish itself.
Medications that shift the balance between estrogen and androgens, including testosterone therapy and anabolic steroids, also commonly cause cycle changes. Anti-seizure medications like valproate and carbamazepine round out the list. If your periods became irregular after starting a new medication, the timing is worth noting.
Elevated Prolactin Without Medication
Prolactin can rise for reasons beyond medication. Small benign tumors on the pituitary gland (called prolactinomas) are a well-known cause. Excess prolactin suppresses the brain’s release of reproductive hormones, reduces the frequency and strength of LH pulses, and can cause infrequent ovulation or no ovulation at all. The result is irregular periods, very light periods, or periods that stop. Breastfeeding raises prolactin through the same pathway, which is why many nursing parents don’t menstruate for months after delivery.
Perimenopause
If you’re in your 40s and your previously regular cycle has become unpredictable, perimenopause is the most likely explanation. This transitional phase typically begins in the 40s, though some people notice changes as early as their 30s or as late as their 50s. During perimenopause, the ovaries gradually produce less estrogen, and ovulation becomes inconsistent. Cycles may become shorter, then longer, then skip months entirely.
The transition ends when you’ve gone 12 consecutive months without a period, which marks menopause. The perimenopausal phase can last anywhere from a few years to over a decade, and cycle irregularity during this time is normal rather than a sign of disease.
The First Few Years After Your First Period
Irregular cycles are expected in adolescence. During the first year after a first period, irregular timing is considered a normal part of the pubertal transition as the hormonal system matures. In the one-to-three-year window after that first period, cycles anywhere from 21 to 45 days apart are within the normal range. That window is wider than the 21-to-35-day range used for adults because the signaling system is still calibrating. If any single cycle goes beyond 90 days during the first year, or if periods haven’t started by age 15, that falls outside the expected range and warrants evaluation.
Patterns Worth Paying Attention To
A single late or early period is rarely meaningful. Bodies respond to travel, illness, a bad week of sleep, or a change in routine, and one off cycle is common. The patterns that point to an underlying cause tend to be persistent: cycles consistently shorter than 21 days or longer than 35 days, fewer than 8 periods in a year, or bleeding that lasts longer than 7 days. Bleeding between periods, bleeding after sex, or sudden changes in a cycle that was previously regular are also worth tracking.
Keeping a record of your cycle length, flow, and symptoms for a few months gives you (and any healthcare provider) much better data to work with than trying to recall patterns from memory. Many of the causes above are straightforward to identify with blood tests for thyroid function, prolactin, and reproductive hormones, or with an ultrasound to check for structural issues like fibroids or polyps.