A “regular” period can fall anywhere between 21 and 35 days apart, lasting 2 to 7 days. If the gap between your cycles varies by more than 9 days from month to month, or your period consistently falls outside that 21-to-35-day window, it qualifies as irregular. The reasons range from completely normal life stages to underlying health conditions worth investigating.
Your Age Plays a Bigger Role Than You Think
The most common reason for irregular periods is simply where you are in your reproductive life. In the first few years after your period starts, the hormonal signaling system between your brain and ovaries is still maturing. Cycles can swing from under 20 days to over 45 days during this time. By the third year after a first period, only 60 to 80 percent of cycles have settled into the typical 21-to-34-day adult range. So if you’re a teenager or in your early twenties and your period is unpredictable, that’s often the explanation.
On the other end, perimenopause brings a similar kind of unpredictability. Most people notice changes in their 40s, but some see shifts as early as their mid-30s. In early perimenopause, cycles start drifting by seven or more days from what’s been normal for you. In late perimenopause, gaps of 60 days or more between periods are common. This transition can last several years before periods stop entirely.
Stress Can Shut Down Your Cycle
Stress doesn’t just make you feel lousy. It directly interferes with the hormonal chain reaction that triggers ovulation. When your body produces high levels of its main stress hormone, cortisol, it reduces the strength of the hormonal signals your pituitary gland sends to your ovaries. Research in endocrinology has shown cortisol can cut the strength of those signals roughly in half within just one hour, with maximum suppression happening within two hours. Without a strong enough signal, your ovaries may not release an egg on schedule, and your period either comes late or skips entirely.
This doesn’t require a major life crisis. Chronic, low-grade stress from work pressure, sleep deprivation, or emotional strain can keep cortisol elevated enough to throw off your cycle over time.
Undereating and Overexercising
Your body treats reproduction as optional when energy is scarce. Losing 10 to 15 percent of your body weight in a short period can cause your period to disappear for six months or longer, a condition sometimes called weight loss-related amenorrhea. The old idea that there’s a specific body fat percentage below which periods stop has been largely debunked, since some athletes with very low body fat menstruate normally while others at higher weights lose their periods. What matters more is the energy gap: whether you’re consistently burning far more calories than you’re taking in.
This is common among endurance athletes, people with eating disorders, and anyone combining intense exercise with calorie restriction. The brain essentially dials down the reproductive system to conserve energy for survival functions. Restoring adequate nutrition typically brings cycles back, though it can take months.
Thyroid Problems
Your thyroid gland has a surprisingly direct connection to your menstrual cycle. When thyroid function is low (hypothyroidism), the brain produces less of the hormones that tell your ovaries to mature and release eggs. On top of that, an underactive thyroid often causes a rise in prolactin, a hormone normally associated with breastfeeding, which further suppresses the ovulation signal. The result can be anything from longer, heavier periods to skipped cycles.
An overactive thyroid (hyperthyroidism) can cause lighter or less frequent periods. In both cases, treating the thyroid issue typically restores normal cycles. Thyroid problems are one of the most treatable causes of irregular periods, which is why a thyroid function test is usually one of the first things checked.
PCOS and Hormonal Imbalances
Polycystic ovary syndrome, or PCOS, is one of the most common hormonal conditions in people of reproductive age. It involves higher-than-normal levels of androgens (sometimes called “male hormones,” though everyone produces them). These elevated androgens can prevent your ovaries from releasing an egg each month, leading to long, unpredictable cycles or missed periods altogether. Other signs include acne, thinning hair on the scalp, or excess facial or body hair.
Elevated prolactin from other causes, such as a small benign growth on the pituitary gland called a prolactinoma, can also disrupt the hormonal signals needed for regular ovulation. High prolactin interferes with the maturation of egg follicles and the normal function of the tissue that supports early pregnancy after ovulation.
Birth Control and Coming Off It
Hormonal contraceptives work by overriding your natural cycle, so it’s no surprise that stopping them can leave things disorganized for a while. After discontinuing the pill, a patch, or a hormonal IUD, it typically takes up to three months for ovulation and regular cycles to return. Some people bounce back within a few weeks, while others need several months. If your periods were irregular before you started birth control, that underlying pattern may simply resurface once you stop.
What Gets Checked
If your irregular periods don’t have an obvious explanation, like recent stress, a new exercise routine, or a life stage transition, a healthcare provider will usually start with blood work. The standard panel includes a pregnancy test (the most common reason for a suddenly missed period), thyroid function, prolactin levels, follicle-stimulating hormone (which shows whether your ovaries are responding normally), and sometimes androgen levels if PCOS is suspected.
Imaging may follow if blood work points in a certain direction. An ultrasound can reveal structural issues in the uterus or ovaries, while an MRI might be ordered if a pituitary issue is suspected. In some cases, a thin camera is passed through the cervix to look directly inside the uterus for polyps, fibroids, or scar tissue that could be affecting bleeding patterns.
Signs That Need Prompt Attention
Some patterns of irregularity warrant a closer look sooner rather than later. Missing your period for 90 days or more (when you’re not pregnant, breastfeeding, or in menopause) is considered abnormal. Soaking through a pad or tampon every hour for two to three consecutive hours, passing blood clots larger than a quarter, bleeding between periods, or periods lasting longer than seven days are all signals to get evaluated. The same goes for cycles that were previously steady and suddenly become erratic, or for severe pain during or between periods.