A period is considered late when it arrives five or more days after you expected it based on your usual cycle. If six weeks pass with no bleeding at all, it’s classified as a missed period. While pregnancy is the most common reason, dozens of other factors can delay your cycle, from everyday stress to underlying hormonal conditions. Understanding the most likely causes helps you figure out whether a late period is a one-time blip or something worth investigating.
Pregnancy Is the First Thing to Rule Out
If you’re sexually active and your period is late, pregnancy is the most likely explanation. Home pregnancy tests detect a hormone called hCG in your urine, but their sensitivity varies by brand. Some tests can pick up a pregnancy before a missed period, but for the most reliable result, the FDA recommends testing one to two weeks after you miss your period. Even on the first day of a missed period, 10 to 20 out of every 100 pregnant women will get a false negative, often because of irregular cycles or miscalculating the expected date. If your first test is negative but your period still doesn’t come, test again a week later.
How Stress Delays Your Cycle
Stress is one of the most common non-pregnancy reasons for a late period, and the mechanism is straightforward. When you’re under sustained physical or emotional stress, your body produces more cortisol. Cortisol interferes with the hormonal signaling chain that triggers ovulation. Specifically, it disrupts the pulsing release of a brain signal that tells your ovaries to prepare and release an egg. If ovulation gets pushed back by a week, your period shifts by roughly the same amount.
This doesn’t require a major life crisis. A stretch of poor sleep, a heavy exam period, a cross-country move, or a demanding month at work can be enough. The period typically returns to normal once the stressor resolves, but chronic, ongoing stress can cause repeated delays or stop periods altogether.
Body Weight and Body Fat
Your body needs a certain amount of energy reserves to sustain a menstrual cycle. Research suggests that body fat needs to be above roughly 22% for regular periods. Dropping below that threshold, whether through intense exercise, restrictive eating, or rapid weight loss, can trigger a condition called hypothalamic amenorrhea, where the brain essentially shuts down the reproductive signal because it senses insufficient energy availability.
This is common in competitive athletes, people with eating disorders, and anyone who dramatically cuts calories. On the other end of the spectrum, carrying significantly more body fat can also disrupt the hormonal balance needed for regular ovulation, though the mechanism is different. Excess fat tissue produces extra estrogen, which can interfere with the normal hormonal rhythm of the cycle. In both cases, periods often resume when body composition returns to a range the body interprets as safe for reproduction.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal disorders in women of reproductive age, and irregular or late periods are its hallmark symptom. The condition involves higher-than-normal levels of androgens (often called “male hormones,” though all women produce them), which interfere with regular ovulation. Without consistent ovulation, periods become unpredictable, sometimes arriving every 35 to 60 days, sometimes skipping months entirely.
A PCOS diagnosis typically requires at least two of three features: signs of excess androgens (such as acne, thinning hair, or excess facial or body hair), irregular or absent ovulation, and a characteristic appearance of the ovaries on ultrasound. Other conditions that could explain the symptoms need to be ruled out first. If your periods are consistently more than 35 days apart or you have fewer than eight periods a year, PCOS is worth discussing with a healthcare provider. It’s manageable with lifestyle changes, hormonal treatments, or both.
Thyroid Problems
Your thyroid gland plays a surprisingly large role in menstrual regularity. Thyroid hormones help regulate metabolism across your entire body, including your reproductive system. When the thyroid is underactive (hypothyroidism), the disruption cascades into your cycle. In one study of reproductive-age women, 55% of those with elevated thyroid-stimulating hormone levels had infrequent periods, and higher TSH levels correlated with more severe menstrual irregularity.
An overactive thyroid (hyperthyroidism) can also cause cycle changes, though lighter or shorter periods are more typical than late ones. Thyroid disorders are diagnosed with a simple blood test and are highly treatable. If late periods come alongside fatigue, unexplained weight changes, feeling unusually cold or warm, or changes in your hair and skin, a thyroid check is a reasonable next step.
High Prolactin Levels
Prolactin is the hormone responsible for milk production after childbirth, which is why breastfeeding women often don’t get periods. But prolactin can be elevated outside of pregnancy and breastfeeding too. The most common cause is a small, benign growth on the pituitary gland called a prolactinoma. High prolactin directly inhibits the same brain signal that cortisol disrupts: the pulsing release of the hormone that drives ovulation.
Certain medications can also raise prolactin levels, including some antipsychotics, antidepressants, and anti-nausea drugs. If you’ve started a new medication and your periods have become irregular, that connection is worth raising with whoever prescribed it.
Hormonal Birth Control and Coming Off It
Hormonal contraceptives work by suppressing your natural cycle, so it’s normal for periods to be late or absent while using them, especially with hormonal IUDs, implants, or continuous-use pills. What catches many people off guard is what happens after stopping. It can take your body one to three months to resume regular ovulation after discontinuing hormonal birth control, and for some people it takes longer. This post-pill delay is temporary and doesn’t indicate a fertility problem, but it can be anxiety-inducing if you’re trying to conceive or simply expecting your cycle to snap back immediately.
Perimenopause
If you’re in your mid-40s and noticing that your previously predictable cycle is becoming erratic, perimenopause is the most likely explanation. This transition phase typically begins around age 47 or 48, though it can start earlier. During perimenopause, ovulation becomes inconsistent. You might have a 25-day cycle followed by a 40-day cycle, then skip a month entirely. Periods may also change in flow, becoming heavier or lighter than usual. This phase lasts an average of four to eight years before periods stop completely at menopause, which occurs at a median age of 50 to 52 in most industrialized countries.
Other Common Causes
Several everyday factors can shift your cycle by a few days to a couple of weeks:
- Travel and time zone changes can throw off your internal clock enough to delay ovulation, particularly on long international trips.
- Illness or infection around the time you would normally ovulate can push ovulation back, which delays the entire cycle.
- Significant changes in exercise, like suddenly training for a marathon or starting a demanding new workout program, can suppress ovulation temporarily.
- Recent weight fluctuation in either direction, even five to ten pounds gained or lost quickly, can be enough to shift your cycle.
These causes tend to be self-correcting. Once your body adjusts to the new routine or recovers from the disruption, cycles typically normalize within one to three months.
When a Late Period Needs Medical Attention
A single late period with an obvious explanation, like a stressful month or a bout of the flu, generally isn’t cause for concern. The clinical threshold for investigation is three consecutive missed cycles if your periods are usually regular, or six months without a period if your cycles have always been irregular. At that point, a provider will typically check for pregnancy, thyroid function, prolactin levels, and signs of PCOS or other hormonal imbalances. Most causes of persistently late or absent periods are treatable once identified.