A period is considered late when it arrives five or more days past when you expected it based on your usual cycle length. If you’ve gone more than six weeks without a period, it’s clinically considered missed. Most of the time, a late period has a straightforward explanation, and understanding the most likely causes can help you figure out what’s going on.
Pregnancy Is the Most Common Cause
If you’re sexually active, pregnancy is the first thing to rule out. Home pregnancy tests claim over 99% accuracy when used from the day of your expected period, but that number depends on following the instructions precisely and having enough of the pregnancy hormone in your urine. If you test too early, you can get a false negative. Testing again a week after your missed period gives the hormone more time to build up and makes the result more reliable. Claims that tests can detect pregnancy eight or more days before your period is due are unrealistic.
How Stress Delays Your Period
Stress is one of the most common reasons for a late period in people who aren’t pregnant. When you’re under significant physical or emotional stress, your body produces more cortisol. That cortisol doesn’t act on your reproductive hormones directly. Instead, it targets a group of specialized brain cells that normally send the signals your ovaries need to release an egg. When those signals slow down or stop, ovulation gets delayed, and your period follows suit.
This isn’t limited to extreme stress. A difficult month at work, a cross-country move, grief, sleep deprivation, or even jet lag can be enough to throw off the timing. Once the stressor resolves, most people see their cycle return to normal within one to two months.
Low Body Weight and Intense Exercise
Your body needs a minimum level of body fat to maintain a regular cycle. The key player is leptin, a hormone produced by fat cells. Research has identified a critical leptin threshold: when levels drop below about 1.85 micrograms per liter, menstruation tends to stop. This is why periods commonly disappear in people with eating disorders, extreme calorie restriction, or very high training volumes.
You don’t have to be visibly underweight for this to happen. Runners, dancers, gymnasts, and other athletes sometimes lose their periods while appearing healthy. The issue isn’t just low weight but an energy imbalance where you’re burning significantly more than you’re taking in. Restoring adequate nutrition typically brings the cycle back, though it can take several months.
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 symptom. To be diagnosed, you generally need two of the following three features: higher than normal levels of androgens (often showing up as acne, excess hair growth, or thinning hair), irregular ovulation, and a specific appearance of the ovaries on ultrasound or elevated levels of a hormone called AMH.
If you have both the androgen symptoms and irregular cycles, that’s enough for a diagnosis without imaging. In PCOS, the ovaries often don’t release eggs on a predictable schedule. Without ovulation, there’s no hormonal trigger telling the uterine lining to shed, so periods come late, unpredictably, or not at all. Cycles longer than 35 days or fewer than eight periods per year are considered irregular in adults who are past the first few years of menstruation.
Thyroid Problems
Your thyroid gland has a surprising amount of influence over your menstrual cycle. When the thyroid is underactive (hypothyroidism), your body ramps up production of a brain hormone called TRH to try to stimulate it. The problem is that TRH doesn’t just affect the thyroid. It also triggers the release of prolactin, a hormone best known for milk production. Elevated prolactin interferes with the hormones that drive ovulation, specifically FSH and GnRH, which can delay or stop your period entirely.
Other signs of an underactive thyroid include fatigue, weight gain, feeling cold, dry skin, and constipation. A simple blood test can check your thyroid levels, and treatment typically restores regular cycles.
Stopping Birth Control
If you recently stopped hormonal birth control, you might expect a delay before your natural cycle kicks back in. The reality is more reassuring than many people think. Research tracking women after they stopped oral contraceptives found that the median length of the first post-pill cycle was 30 days, essentially normal. The second cycle was 29 days. There was no significant difference compared to women who hadn’t been on the pill at all.
One exception: women aged 18 to 24 did tend to have a slightly longer first cycle after stopping. And individual variation can be wide, with some people seeing a first cycle as short as 15 days and others waiting up to 82 days. If your period hasn’t returned within three months of stopping contraception, that’s worth investigating further. Injectable contraceptives like the shot are a different story and can suppress ovulation for longer.
Perimenopause
If you’re in your 40s and noticing that your periods are becoming unpredictable, perimenopause is a likely explanation. This transition phase can start in the mid-30s for some people, though most notice changes in their 40s. The hallmark of early perimenopause is a cycle length that shifts by seven days or more from what’s been normal for you. In late perimenopause, gaps of 60 days or more between periods are common.
During this time, ovulation becomes erratic. You might have a 24-day cycle followed by a 40-day cycle. Flow can swing from unusually heavy to barely there. These changes can last anywhere from a few years to a decade before periods stop completely at menopause.
Certain Medications
Several types of medication can delay or stop periods as a side effect. Antipsychotic medications are among the most common culprits, with studies finding that 11 to 35% of women taking them experience missed periods. These drugs can raise prolactin levels, which blocks ovulation through the same mechanism as thyroid problems.
Other medications that may affect your cycle include certain antidepressants, chemotherapy drugs, and some blood pressure medications. If your period became irregular after starting a new medication, that connection is worth discussing with your prescriber. Stopping or switching the medication often resolves the issue.
How Long Is Too Long
A period that’s a week or two late once in a while is usually not a sign of anything serious. Bodies aren’t clocks, and occasional variation is normal. But there are thresholds that signal something worth looking into. If you’ve previously had regular cycles and go three consecutive months without a period, that meets the clinical definition of secondary amenorrhea. If your cycles have always been irregular and you go six months without bleeding, that’s the same threshold.
Other patterns worth paying attention to: cycles consistently shorter than 21 days or longer than 35 days, periods that suddenly become much heavier or more painful than usual, or bleeding between periods. These don’t necessarily mean something is wrong, but they’re the kinds of changes that blood work or imaging can help explain.