Irregular periods have dozens of possible causes, ranging from completely normal life stages to hormonal conditions worth investigating. A “regular” cycle falls between 21 and 35 days, with some natural variation from month to month. If your cycles consistently differ by seven or more days in length, or you’re skipping periods entirely, something is shifting your hormonal balance.
The most common culprits are age-related changes, thyroid problems, polycystic ovary syndrome (PCOS), stress, and energy imbalance from diet or exercise. Here’s how each one works and what to look for.
Your Age Matters More Than You Think
If you’re a teenager whose periods started in the last few years, irregularity is expected. After your first period, the hormonal feedback loop between your brain and ovaries takes time to mature. During this window, cycles can be long, short, or unpredictable. That said, persistent irregularity in adolescents can sometimes signal an underlying issue like PCOS or thyroid disease, so it’s worth tracking your cycles even in the early years.
On the other end of the spectrum, perimenopause brings a return of unpredictable cycles, sometimes starting in your late 30s but more commonly in your 40s. In early perimenopause, your cycle length starts shifting by seven or more days compared to what’s been normal for you. In late perimenopause, gaps of 60 days or more between periods are typical. These changes can stretch over several years before periods stop entirely at menopause.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal conditions in women of reproductive age, and irregular or absent periods are its hallmark. The core issue is an excess of androgens, sometimes called “male hormones,” though everyone produces them. Elevated testosterone or related hormones can disrupt the normal monthly process of releasing an egg, which is what keeps your cycle on schedule.
Doctors diagnose PCOS by looking for a combination of irregular cycles, signs of excess androgens (acne, excess hair growth, thinning hair on the scalp, or elevated levels on a blood test), and a specific pattern on ultrasound where the ovaries contain 20 or more small follicles. You don’t need all three. Two out of three, after ruling out other causes, is enough for a diagnosis. A blood test called AMH, which reflects ovarian follicle count, can sometimes substitute for an ultrasound in adults.
PCOS often comes with insulin resistance, which means your body struggles to manage blood sugar efficiently. This connection is why weight management and blood sugar control through diet and exercise can improve cycle regularity for many people with the condition, even without medication.
Thyroid Problems
Your thyroid gland, the butterfly-shaped gland at the front of your neck, has a surprisingly direct connection to your menstrual cycle. Thyroid hormones influence the brain signals that trigger ovulation, and when thyroid function drops (hypothyroidism), the disruption cascades through your reproductive system in multiple ways.
An underactive thyroid raises levels of prolactin, a hormone normally associated with breastfeeding. Elevated prolactin suppresses the brain’s release of the hormones that drive your cycle. On top of that, the thyroid-stimulating hormone (TSH) your body produces in response to low thyroid function can actually interact with receptors on the ovaries that are meant for other hormones, further confusing the system. Estrogen levels also track closely with thyroid hormone levels, so when one drops, the other tends to follow.
An overactive thyroid (hyperthyroidism) can cause lighter or less frequent periods. Either direction of thyroid dysfunction can throw off your cycle, which is why a simple thyroid blood test is one of the first things a doctor will check when you report irregular periods.
Elevated Prolactin
Even without a thyroid problem, prolactin levels can rise on their own and disrupt your cycle. This condition, called hyperprolactinemia, can cause irregular periods, missed periods, or milky nipple discharge unrelated to pregnancy or breastfeeding. Common causes include certain medications (especially some antidepressants and anti-nausea drugs) and benign pituitary growths. It’s diagnosed with a blood test and is usually very treatable.
Stress and Energy Balance
Your brain monitors your body’s energy status closely, and when it senses you’re under significant physical or emotional stress, or not taking in enough fuel, it can shut down ovulation to conserve resources. This is your body’s way of saying “now is not a safe time for pregnancy.”
For athletes and people who exercise intensely, the threshold matters. Research on energy availability in female athletes shows that when calorie intake minus exercise expenditure drops below about 30 calories per kilogram of fat-free body mass per day, roughly half will develop menstrual dysfunction. At 20 or below, periods often stop entirely. The optimal level for maintaining a regular cycle is at or above 45 calories per kilogram of fat-free mass per day. You don’t need to calculate this precisely to get the message: if you’ve ramped up training, started a restrictive diet, or lost weight rapidly, and your periods have become irregular or disappeared, the two are almost certainly connected.
Chronic psychological stress works through a similar pathway. Sustained stress hormones interfere with the same brain signals that control your cycle, which is why periods can become unpredictable during major life upheavals, grief, or prolonged anxiety.
Coming Off Hormonal Birth Control
If you recently stopped taking hormonal contraception, a delay before your periods return is normal. In a study of over 300 women who stopped oral contraceptives, 89% began menstruating within 60 days. Only about 7% took 180 days or longer. In rare cases (about 2%), the gap stretched much further, but all women in the study eventually resumed cycling on their own, with the longest delay being about 18 months.
Hormonal IUDs, implants, and injections can have different timelines. Injectable contraceptives tend to have the longest return-to-normal window. If your periods haven’t returned within three months of stopping the pill, or six months after stopping injections, it’s reasonable to get checked, since the delay could be masking another issue like PCOS or thyroid dysfunction that was hidden while you were on contraception.
Other Contributing Factors
Several less common but important causes deserve mention. Significant weight gain or loss can shift estrogen levels enough to disrupt your cycle, since fat tissue produces estrogen. Uterine fibroids or polyps can cause heavy or prolonged bleeding that mimics irregularity. Uncontrolled diabetes and certain chronic illnesses also affect cycle regularity. And while rare, premature ovarian insufficiency (early loss of ovarian function before age 40) can present as increasingly irregular or absent periods.
Signs That Need Prompt Attention
Some patterns of irregular bleeding warrant a quicker call to your doctor. If you’re soaking through a pad or tampon every hour for more than two hours straight, especially with dizziness, lightheadedness, or shortness of breath, that’s an emergency. Bleeding that has been abnormal for six months or more qualifies as a chronic issue and should be evaluated. Periods that suddenly stop for 90 days or more (outside of pregnancy, breastfeeding, or recent contraception use) also merit investigation.
Tracking your cycles with a simple app or calendar gives your doctor far more useful information than trying to recall patterns from memory. Note the start date, how long bleeding lasts, and how heavy it is. Even a few months of data can help pinpoint whether the irregularity follows a recognizable pattern tied to one of the causes above.