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, that counts as a missed period. While pregnancy is the most common reason, it’s far from the only one. Stress, weight changes, hormone imbalances, and even certain medications can all push your cycle off schedule.
Pregnancy
This is the first thing most people think of, and for good reason. After a fertilized egg implants in the uterus, your body starts producing a hormone called hCG. That hormone is what home pregnancy tests detect in your urine. It becomes measurable about 10 days after conception, but testing too early raises the chance of a false negative. The most reliable time to take a home test is the day your period was due or later. By that point, hCG levels are typically high enough for any standard test to pick up.
If you get a negative result but your period still hasn’t arrived a week later, it’s worth testing again. HCG roughly doubles every two to three days in early pregnancy, so a test that missed it on day one may catch it several days later.
Stress and Your Hormonal Signals
Your brain controls the timing of your cycle through a chain of hormonal signals that starts in the hypothalamus, a small region that acts as a control center for reproduction. Physical or emotional stress can disrupt those signals, delaying or even skipping ovulation altogether. When ovulation is delayed, your period shifts later too, because the countdown to bleeding only begins after ovulation occurs.
This isn’t limited to major life crises. Sleep deprivation, illness, travel across time zones, or a particularly demanding stretch at work can all be enough to throw things off. The delay is usually temporary: once the stressor passes, most cycles return to their normal rhythm within one to two months.
Low Energy Availability
Your body needs a minimum amount of energy left over after exercise to keep your reproductive system running. Researchers define this as “energy availability,” meaning the calories you eat minus the calories you burn during exercise, divided by your lean body mass. In a study of women aged 18 to 30, reproductive hormone pulses dropped significantly when energy availability fell below 30 kilocalories per kilogram of lean body mass per day.
You don’t have to be underweight for this to happen. A combination of increased training and modest calorie restriction can cross this threshold even at a normal body weight. Conversely, rapid weight gain can also disrupt cycles by shifting estrogen levels. The pattern is the same in both directions: your hypothalamus senses that conditions aren’t ideal for supporting a pregnancy and dials back the hormonal signals that trigger ovulation.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal conditions in women of reproductive age, and irregular or late periods are a hallmark. In PCOS, the ovaries produce higher than normal levels of androgens (sometimes called “male hormones,” though everyone makes them). That excess can prevent eggs from maturing and releasing on schedule, which delays or skips periods entirely.
Diagnosis typically requires two of three features: irregular cycles, elevated androgen levels (which can show up as excess facial or body hair, acne, or thinning hair on the scalp), and a characteristic appearance of the ovaries on ultrasound. Updated international guidelines from 2023 now also allow a blood test measuring anti-Müllerian hormone as an alternative to ultrasound in adults. Excess facial or body hair alone is considered a strong predictor of elevated androgens, while acne without other signs is a weaker one.
PCOS doesn’t go away on its own, but the cycle irregularity it causes is manageable with lifestyle changes or medication depending on severity and whether pregnancy is a goal.
Thyroid Problems
Your thyroid gland sets the metabolic pace for nearly every system in your body, including reproduction. An underactive thyroid (hypothyroidism) tends to lower estrogen and progesterone, which can shorten or delay cycles. An overactive thyroid (hyperthyroidism) can cause lighter or less frequent periods.
Research in premenopausal women has found that lower levels of free T4 (the active thyroid hormone) were associated with slightly shorter cycles, while higher levels correlated with slightly longer ones, with the difference driven by the first half of the cycle before ovulation. These shifts were subtle within the normal thyroid range, but when thyroid function swings further out of balance, the effects on your period become more noticeable. A simple blood test can check whether your thyroid is contributing to cycle changes.
Medications That Raise Prolactin
Prolactin is the hormone responsible for milk production, but it also suppresses the reproductive signals needed for regular ovulation. Certain medications can push prolactin levels far above normal, sometimes to ten times the usual concentration. Antipsychotic medications are the most common culprits. Studies show that 17 to 78 percent of women taking these drugs experience missed periods, with or without breast discharge.
The mechanism involves dopamine, a brain chemical that normally keeps prolactin in check. Most antipsychotics block dopamine receptors, which removes the brake on prolactin production. Some newer antipsychotics have less effect on prolactin, and case reports show that women who switched to these alternatives or discontinued the medication resumed regular cycles. Serotonin-based medications like certain antidepressants can also raise prolactin to a lesser degree through a different pathway, though missed periods from SSRIs alone are less common.
If your periods changed after starting a new medication, that connection is worth mentioning to your prescriber. Adjusting the drug or dose often resolves the problem.
Perimenopause
If you’re in your 40s and your previously predictable cycle is becoming erratic, perimenopause is a likely explanation. This transitional phase before menopause typically begins in the mid-40s, though some women notice changes as early as their mid-30s. During perimenopause, estrogen and progesterone levels fluctuate unpredictably rather than following their usual monthly pattern. Some cycles, ovulation happens late. Other cycles, it doesn’t happen at all, and your period may skip entirely.
You might have a 25-day cycle one month and a 45-day cycle the next. Periods may also become heavier or lighter than usual. This phase lasts an average of four to eight years before periods stop completely. No single test definitively diagnoses perimenopause; the pattern of increasingly irregular cycles in the right age range is the main indicator.
Early Cycles and Adolescence
For teenagers, irregular periods are the norm rather than the exception. It can take two to three years after a first period for the hormonal feedback loop to mature enough to produce regular cycles. Skipping a month or arriving a week late during this window is common and rarely signals a problem. The American College of Obstetricians and Gynecologists recommends evaluation if a teen hasn’t started menstruating by age 15, or if there are no signs of breast development by age 13.
Other Common Triggers
Several everyday factors can shift your cycle by a few days to a few weeks:
- Illness or infection: A flu, COVID, or any significant illness around the time you’d normally ovulate can delay it, pushing your period back proportionally.
- Starting or stopping hormonal birth control: After stopping the pill, patch, or hormonal IUD, it can take one to three months for your natural cycle to re-establish itself. Starting a new method can also cause spotting or timing changes in the first few months.
- Breastfeeding: Prolactin stays elevated while nursing, which suppresses ovulation. Periods often return irregularly as feeding frequency decreases.
- Shift work or jet lag: Your circadian rhythm influences reproductive hormones. Disrupting it through overnight shifts or long-distance travel can delay ovulation.
How Long a Late Period Can Wait
A period that’s a few days late after a stressful month or a schedule change is rarely cause for alarm. The general guideline from the American College of Obstetricians and Gynecologists is to seek evaluation if your period stops for more than three months without a clear explanation like pregnancy or a known medication change. If you’re consistently having cycles longer than 35 to 40 days, or if late periods are accompanied by new symptoms like excess hair growth, significant weight changes, or hot flashes, those patterns are worth investigating sooner rather than later.