Irregular periods have a wide range of causes, from everyday stress and lifestyle changes to hormonal conditions that need treatment. A normal menstrual cycle falls between 21 and 35 days, so if your cycles consistently fall outside that window, vary by more than 20 days from one cycle to the next, or disappear entirely for stretches of time, something is shifting your hormonal balance.
The good news is that most causes of irregular periods are identifiable and manageable. Understanding what counts as irregular, and what the most common culprits are, can help you figure out your next step.
What Counts as an Irregular Period
Cycles shorter than 21 days or longer than 35 days are considered outside the normal range. But irregularity isn’t just about cycle length. If your cycles swing by more than 20 days from month to month (say, 25 days one cycle and 48 the next), that’s clinically irregular even if some individual cycles land in the normal range. Bleeding that lasts longer than 8 days on a regular basis, or periods that consistently last fewer than 2 days, also fall into this category.
Keep in mind that the occasional off cycle is common. A period that’s a few days late once or twice a year, especially around travel or illness, doesn’t necessarily signal a problem. The pattern matters more than any single cycle.
PCOS: The Most Common Hormonal Cause
Polycystic ovary syndrome is one of the leading reasons for irregular or missing periods in people of reproductive age. In PCOS, the brain sends too-frequent signals to the pituitary gland, which creates an imbalance: too much of the hormone that stimulates androgen (male hormone) production, and not enough of the hormone that helps eggs mature. The result is that follicles in the ovary start developing but never fully mature or release an egg. Without ovulation, your body doesn’t get the hormonal signal to trigger a normal period.
Insulin resistance often makes this worse. When insulin levels stay elevated, they can further amplify androgen production, creating a feedback loop that deepens the hormonal imbalance. This is why PCOS frequently shows up alongside acne, excess hair growth on the face or body, thinning hair on the scalp, and difficulty losing weight. If your irregular periods come with any of these signs, PCOS is worth investigating. Diagnosis typically involves blood tests measuring testosterone and other androgens, sometimes combined with an ultrasound or a blood marker called AMH.
Thyroid Problems
Your thyroid gland plays a direct role in regulating your cycle. Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can make periods heavier, lighter, more frequent, less frequent, or absent altogether. The thyroid hormones help maintain the balance of reproductive hormones that trigger ovulation, so when thyroid output is off, ovulation can become unpredictable.
Thyroid issues are easy to screen for with a simple blood test measuring TSH (thyroid-stimulating hormone). If you’re experiencing irregular periods along with symptoms like unexplained fatigue, weight changes, sensitivity to cold or heat, or hair thinning, a thyroid panel is a reasonable place to start.
Stress and the Cortisol Connection
Chronic stress doesn’t just make you feel terrible. It can directly suppress your reproductive hormones. When your body produces sustained high levels of cortisol (the primary stress hormone), cortisol acts on the brain to slow down the pulses of the signal that kicks off your entire hormonal cascade. Fewer pulses means less stimulation of the ovaries, which can delay or prevent ovulation. Research has shown that sustained cortisol elevation can reduce the frequency of these hormonal pulses by as much as 70% during the fertile phase of the cycle.
This doesn’t require a catastrophic life event. Ongoing work pressure, sleep deprivation, caregiving stress, or chronic anxiety can keep cortisol elevated long enough to disrupt your cycle. If your periods became irregular around the same time your stress levels increased, the connection is likely real.
Undereating and Overexercising
Your body needs a minimum level of energy availability to maintain a menstrual cycle. When calorie intake drops too low relative to what you’re burning, especially through intense exercise, the brain can shut down reproductive signaling entirely. This is called hypothalamic amenorrhea, and it’s the body’s way of conserving energy by deprioritizing reproduction.
There’s no single body fat percentage or calorie threshold that guarantees this will happen, but the risk increases as energy availability decreases. It’s common in competitive athletes, people following very restrictive diets, and those with eating disorders, but it can also happen to someone who’s simply increased their exercise significantly while not eating enough to compensate. Periods typically return once energy balance is restored, though recovery can take months.
Perimenopause
If you’re in your 40s and your previously predictable periods have started behaving unpredictably, perimenopause is a likely explanation. This transition typically begins in the mid-40s, though it can start as early as the mid-30s. It lasts an average of four years but can stretch to eight.
During perimenopause, estrogen levels don’t decline in a smooth, gradual curve. They fluctuate wildly, sometimes spiking higher than normal before dropping again. This rollercoaster throws off the balance with progesterone, leading to cycles that may be shorter or longer than usual, heavier or lighter, or skipped entirely. The first sign for most people is simply that periods stop arriving on schedule. This is a normal part of aging, not a medical problem, though the symptoms that come with it (hot flashes, sleep disruption, mood changes) are worth addressing if they affect your quality of life.
Medications That Can Alter Your Cycle
A surprising number of medications can disrupt or stop periods entirely. The most well-known are hormonal contraceptives, but the list extends far beyond that:
- Antipsychotics (used for conditions like schizophrenia and bipolar disorder) frequently interfere with the hormonal signals that drive ovulation.
- Antidepressants, including SSRIs and older tricyclic antidepressants, can affect cycle regularity.
- Opioid pain medications suppress reproductive hormones with regular use.
- Anti-seizure medications used for epilepsy or nerve pain can alter cycle timing.
- Certain blood pressure medications and drugs used for digestive problems have also been linked to missed periods.
If your periods became irregular after starting a new medication, that’s worth raising with whoever prescribed it. There may be alternative options that don’t carry the same side effect.
Other Possible Causes
Several other conditions can lead to irregular periods. Uterine fibroids or polyps often cause heavy or prolonged bleeding. Premature ovarian insufficiency (when the ovaries stop functioning normally before age 40) causes irregular or absent periods along with symptoms similar to menopause. Uncontrolled diabetes, significant weight gain or loss, and conditions affecting the pituitary gland can all disrupt the hormonal chain of command that controls your cycle.
What to Track Before Your Appointment
If you’re planning to see a healthcare provider about irregular periods, arriving with data makes diagnosis faster and more accurate. At minimum, record the start and end date of each period and note whether bleeding is heavy, moderate, or light. Even two to three months of tracking is useful.
Beyond that, monitoring a few additional signals can help pinpoint what’s going on. Tracking your basal body temperature (taken with a digital thermometer first thing each morning before getting out of bed) can reveal whether you’re ovulating: a slight temperature rise that stays elevated for about two weeks suggests ovulation occurred. Monitoring changes in cervical mucus throughout your cycle provides similar clues. Ovulation test strips, which measure a surge in luteinizing hormone in your urine, can confirm whether and when you’re ovulating. If you’re consistently not ovulating despite having some bleeding, that’s an important finding that points toward conditions like PCOS.
As a general guideline, cycles consistently shorter than 21 days, longer than 40 days, or lasting more than 8 days per period are worth medical evaluation. The same goes for periods that suddenly become much heavier than your normal, or if your periods stop for three or more months without an obvious explanation like pregnancy.