A period is considered late when it arrives more than 7 days after your expected date. After 6 weeks with no period at all, it’s generally classified as a missed period rather than a late one. Pregnancy is the most obvious reason, but dozens of other factors can push your cycle off schedule, from stress and weight changes to hormonal conditions you may not know you have.
Pregnancy
If you’re sexually active, pregnancy is the first thing to rule out. Most home pregnancy tests can detect the pregnancy hormone (hCG) starting the day your period is due, with some sensitive tests picking it up as early as 6 days before a missed period. If you get a negative result but your period still hasn’t arrived after a few more days, test again. hCG levels roughly double every 48 hours in early pregnancy, so a test taken too early can give a false negative.
Stress and Sleep Disruption
Your menstrual cycle is regulated by a hormonal chain that starts in your brain. Chronic stress, whether emotional, physical, or psychological, can disrupt the signals your brain sends to trigger ovulation. When ovulation is delayed, your period is delayed along with it. This is one of the most common and most overlooked reasons for a late period.
Major life events like a move, job loss, grief, or even exam season can be enough. Sleep disruption plays a similar role. Shift work, jet lag, or consistently poor sleep can interfere with the same hormonal signals. In most cases, your cycle returns to normal once the stressor resolves, though it can take a cycle or two to recalibrate.
Significant Weight Changes
Your body needs a minimum level of body fat to sustain a regular menstrual cycle. Women generally need at least 22% body fat to menstruate regularly, and below 17%, periods may stop entirely. Rapid weight loss, restrictive dieting, or eating disorders can drop body fat below these thresholds, and your body essentially shuts down reproduction as a protective measure.
Weight gain can also disrupt your cycle. Excess body fat increases estrogen production, which can throw off the balance between estrogen and progesterone that governs ovulation. The result is the same: late, irregular, or skipped periods.
Intense Exercise
Heavy training without adequate nutrition creates an energy deficit that your body interprets as a crisis. This condition, called relative energy deficiency in sport (RED-S), happens when you burn more calories than you take in, whether intentionally or accidentally through intense exercise without enough food. Signs include fatigue, abnormal or absent periods, mental health changes, and increased bone and muscle injuries. Over time, prolonged low energy availability can lead to bone loss.
This isn’t limited to elite athletes. Anyone who ramps up their exercise routine significantly, starts training for a marathon, or combines heavy workouts with calorie restriction can experience it. The fix is closing the gap between energy intake and energy output, not necessarily exercising less.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal disorders in women of reproductive age, and irregular periods are its hallmark symptom. In PCOS, the body produces higher-than-normal levels of androgens (often called “male hormones,” though all women produce them in small amounts). These elevated androgens interfere with ovulation, leading to cycles that are unpredictable, infrequent, or absent.
Insulin resistance plays a central role. When the body doesn’t respond well to insulin, it compensates by producing more, which in turn drives the ovaries and adrenal glands to produce even more androgens. This creates a cycle of worsening symptoms. Beyond late periods, PCOS often shows up as acne, excess facial or body hair, thinning hair on the scalp, and difficulty losing weight. Diagnosis requires at least two of three findings: signs of high androgens, ovulatory dysfunction, or polycystic-appearing ovaries on ultrasound.
Thyroid Problems
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can throw off your menstrual cycle. An underactive thyroid is the more common culprit. It can cause your body to produce excess prolactin, the same hormone responsible for breast milk production. Too much prolactin can prevent ovulation entirely, which delays or stops your period.
Thyroid disorders are relatively easy to detect with a blood test and highly treatable. If your periods have become irregular and you’re also noticing unexplained fatigue, weight changes, hair thinning, or feeling unusually cold or warm, thyroid function is worth checking.
Medications That Affect Your Cycle
Several types of medication can delay or stop periods as a side effect. Hormonal birth control is the most obvious, particularly when starting, stopping, or switching methods. After stopping the pill, it can take several months for your natural cycle to resume.
Beyond birth control, other medications that commonly interfere with menstruation include antipsychotics, certain antidepressants, blood pressure medications, stomach acid blockers, and opiates. These drugs can raise prolactin levels, which suppresses ovulation the same way thyroid dysfunction does. Chemotherapy and radiation therapy can also disrupt or permanently alter menstrual cycling, depending on the type and duration of treatment. If your periods became irregular after starting a new medication, that connection is worth discussing with your prescriber.
Perimenopause
If you’re in your 40s and your periods have started arriving on an unpredictable schedule, perimenopause is a likely explanation. This transitional phase typically begins in the mid-40s but can start as early as the mid-30s. It lasts an average of 8 to 10 years before menopause, during which your body gradually produces less of the hormones that drive ovulation.
During perimenopause, cycles may become longer, shorter, heavier, lighter, or skip months entirely. There’s no single test that confirms it definitively. Checking levels of follicle-stimulating hormone (FSH) can offer clues, since consistently high FSH suggests you’re approaching menopause, but hormone levels fluctuate so much during this phase that a single test can be misleading.
Other Causes Worth Knowing
A few less common but real causes include breastfeeding (prolactin levels stay elevated while nursing, which can suppress ovulation for months), recent illness or surgery (your body may delay ovulation while recovering), and conditions affecting the pituitary gland or hypothalamus, which are the brain structures that control the entire hormonal cascade behind your cycle. Chronic conditions like uncontrolled diabetes and celiac disease can also contribute to irregular periods.
How Many Missed Periods Should Concern You
An occasional late period, especially one you can tie to a stressful month, travel, or a lifestyle change, is usually nothing to worry about. But if your period stops for more than 3 consecutive months without explanation, that warrants evaluation regardless of your age. For teens who haven’t started their period by age 15, or who show no signs of breast development by age 13, an evaluation is also recommended. Regular periods are a sign that your hormonal system is working properly, so persistent irregularity is worth investigating even when the cause turns out to be benign.