Irregular periods have a wide range of causes, from completely normal life stages to hormonal conditions worth investigating. A cycle is considered regular when it falls between 21 and 34 days for adults, with roughly the same length from month to month. If your cycles swing wildly in length, come fewer than every 21 days, stretch beyond 35 days, or disappear for 90 days or more, something is shifting your hormonal balance.
The most common reasons fall into a handful of categories: your age, your hormones, your body’s energy balance, and medications you may be taking. Here’s what could be behind the unpredictability.
Your Age Plays a Bigger Role Than You Think
If you’re a teenager or in your early twenties, irregular cycles may simply reflect a body that hasn’t finished calibrating. In the first three years after a first period, long and unpredictable cycles are the norm. By the third year, only 60% to 80% of cycles settle into the typical 21-to-34-day adult range. A person’s individual “normal” cycle length often isn’t fully established until around age 19 or 20. That said, going more than 90 days without a period at any age is uncommon enough to warrant a closer look, even during adolescence.
On the other end of the spectrum, the transition to menopause (perimenopause) brings a second wave of irregularity. In the years leading up to menopause, average cycle length creeps upward: about 35 days at three years out, roughly 45 days at two years out, and a striking 80 days in the final year before periods stop entirely. During that last year, most women spend three-quarters of their time in cycles longer than 40 days. These shifts can start in your early-to-mid forties and sometimes even your late thirties. If you’re in that age window and your cycles are getting longer or less predictable, perimenopause is a likely explanation.
PCOS and Hormonal Imbalances
Polycystic ovary syndrome is one of the most common hormonal causes of irregular periods. It involves higher-than-normal levels of androgens (sometimes called “male hormones,” though all bodies produce them). Those elevated androgens disrupt the normal signaling that triggers ovulation each month. When ovulation doesn’t happen on schedule, your period either arrives late, arrives unpredictably, or skips entirely.
PCOS often comes with other recognizable signs: excess facial or body hair, persistent acne, oily skin, and sometimes ovarian cysts visible on ultrasound. It’s also a metabolic condition. Many people with PCOS develop insulin resistance, meaning their cells don’t respond efficiently to insulin, which further fuels androgen production in a self-reinforcing cycle. This raises long-term risk for type 2 diabetes and cardiovascular problems, which is why a new PCOS diagnosis typically prompts blood work to check insulin and metabolic health markers.
Thyroid disorders are another hormonal culprit. Both an underactive and overactive thyroid can throw off cycle timing, because thyroid hormones interact directly with the reproductive hormone chain. A simple blood test measuring thyroid-stimulating hormone (TSH) can identify this quickly. Similarly, elevated prolactin, a hormone normally involved in milk production, can suppress ovulation and cause missed periods. High prolactin sometimes points to a small, benign growth on the pituitary gland, which is treatable.
When Your Body Doesn’t Have Enough Fuel
Your brain monitors your body’s energy status closely, and when it detects a shortfall, it deprioritizes reproduction. This is called hypothalamic amenorrhea, and it happens when calorie intake consistently falls short of what your body needs, especially when combined with heavy exercise. The hypothalamus, the part of the brain that orchestrates your cycle, essentially dials down the hormonal signals that trigger ovulation.
This doesn’t require an eating disorder, though it’s common in that context. Athletes, dancers, and anyone restricting calories while training hard can cross the threshold. Research on young female athletes found that body fat percentage relative to age is a stronger predictor of lost or infrequent periods than BMI or weight alone. Athletes whose body fat dropped to roughly one standard deviation below average for their age had significantly higher rates of missed periods. When trying to restore a regular cycle, reaching a body fat level at or above that threshold was a more reliable target than hitting a specific number on the scale.
Rapid weight loss, chronic undereating, and extreme physical stress all trigger the same response. If your periods disappeared or became erratic after a significant change in diet, exercise, or body weight, energy availability is the first thing to consider.
Hormonal Birth Control and Medications
Starting, switching, or stopping hormonal contraceptives is one of the most frequent causes of unexpected bleeding patterns. With IUDs, spotting and irregular bleeding in the first few months are normal and typically resolve within two to six months. With the implant, the bleeding pattern you experience in the first three months tends to be the pattern you’ll have going forward, so early irregularity may persist.
Stopping hormonal birth control can also leave cycles erratic for a stretch. The synthetic hormones in contraceptives suppress your body’s own hormonal cycling, and it can take several months for your natural rhythm to resume. This is more pronounced after long-acting methods like injections, where it may take six months or more for regular ovulation to return.
Other medications can interfere too. Certain antipsychotics raise prolactin levels, some anti-seizure drugs affect hormone metabolism, and long-term corticosteroid use can suppress ovulation. If your periods changed after starting a new medication, that connection is worth exploring.
Stress, Sleep, and Other Triggers
Psychological stress acts on the same brain pathway as caloric deficit. Sustained, high-level stress can suppress the hormonal pulses that drive ovulation, delaying or skipping a period. A single acutely stressful month (a death in the family, a major move, job loss) can push one cycle later than expected without signaling a deeper problem. Chronic, ongoing stress is more likely to create a pattern of irregularity.
Significant changes to your sleep schedule, night shift work, and jet lag can also nudge cycles off track. The hormones that regulate your cycle are sensitive to your circadian rhythm, so disrupting your internal clock ripples into reproductive timing. These effects tend to be mild and resolve when the disruption does.
What Testing Looks Like
If irregular cycles persist and aren’t explained by an obvious life change, a healthcare provider will typically start with blood work. A pregnancy test comes first, regardless of how unlikely you think it is. From there, standard panels check thyroid function (TSH), ovarian function (follicle-stimulating hormone), prolactin levels, and androgen levels if PCOS is suspected.
If blood work doesn’t point to a clear answer, imaging may follow. A pelvic ultrasound can reveal ovarian cysts, structural abnormalities, or signs consistent with PCOS. In some cases, a provider may prescribe a short course of a hormonal medication to see whether it triggers a withdrawal bleed. If it does, that confirms your body is producing estrogen but not ovulating. If it doesn’t, the issue may involve lower estrogen levels, as seen in hypothalamic amenorrhea or premature ovarian insufficiency.
For cases that remain unexplained after blood work and imaging, a hysteroscopy (a thin camera passed through the cervix to view the inside of the uterus) can check for structural causes like polyps, fibroids, or scarring that might be affecting bleeding patterns.
Tracking What Matters
Before any appointment, the most useful thing you can do is track your cycles for at least two to three months. Record the first day of each period, how many days bleeding lasts, and any notable symptoms like pain, heavy flow, or spotting between periods. Apps make this easy, but a simple calendar works just as well. This data gives a provider far more to work with than a vague sense that things have been “off.” It can also reveal patterns you might not notice otherwise, like cycles that are consistently long rather than truly random, which points toward different causes than cycles that are wildly variable in both directions.