A late period can be caused by pregnancy, stress, hormonal conditions, weight changes, certain medications, and several other factors. A normal menstrual cycle ranges from 21 to 35 days, so “late” depends on what’s typical for you. If your period is consistently arriving outside that window, or if it stops for three months or longer, something is worth investigating.
Stress and Your Cycle
Stress is one of the most common reasons for a late period, and it doesn’t have to be dramatic. A demanding stretch at work, poor sleep, travel across time zones, or emotional upheaval can all delay ovulation. Your brain’s hormonal signaling system is sensitive to stress hormones, and when those spike, the chain of signals that triggers ovulation can slow down or pause entirely. Since your period arrives roughly two weeks after ovulation, a delay in ovulation pushes your whole cycle back.
This type of disruption is usually temporary. Once the stressor resolves or your body adjusts, cycles tend to normalize on their own within one to two months.
Pregnancy
The most obvious explanation for a missed period is pregnancy. Standard home pregnancy tests detect the pregnancy hormone in urine at a sensitivity of about 25 IU/L, which is typically reliable by the first day of your expected period. If you test too early, you may get a false negative because hormone levels haven’t risen high enough yet. If your first test is negative but your period still hasn’t come a week later, testing again gives a more definitive answer.
Weight Changes and Exercise
Your body needs a certain level of energy availability to sustain a regular cycle. When you’re in a significant caloric deficit, whether from heavy exercise, restrictive eating, or rapid weight loss, your brain can suppress the hormonal signals that drive ovulation. This is called hypothalamic amenorrhea, and it’s especially common in endurance athletes, dancers, and people with eating disorders. Research shows that even a caloric deficit without actual fat loss can trigger this hormonal suppression by lowering leptin, a hormone produced by fat cells that helps regulate reproductive function.
In one study, women who had lost their periods had an average body fat percentage of about 21.5%, compared to 25% in women with normal cycles. But there’s no single cutoff that applies to everyone. Some women lose their periods at a higher body fat percentage if their energy intake doesn’t match their activity level. On the other end of the spectrum, carrying significantly more weight can also disrupt cycles by altering estrogen and insulin levels, which interfere with ovulation.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal conditions affecting menstrual regularity. It involves a combination of irregular or absent ovulation, elevated levels of androgens (sometimes called “male hormones,” though everyone produces them), and in some cases, multiple small cysts on the ovaries visible on ultrasound. You don’t need all three features for a diagnosis. Two out of three is the standard used by most clinicians.
With PCOS, the hormonal imbalance prevents eggs from maturing and being released on a regular schedule. This means periods can be unpredictable, arriving weeks late or skipping months entirely. Other signs include acne, excess facial or body hair, thinning hair on the scalp, and difficulty losing weight. PCOS is manageable with lifestyle changes and, when needed, hormonal treatments that help regulate cycles.
Thyroid Problems
Your thyroid gland plays a direct role in menstrual regulation. Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can make your periods lighter, heavier, irregular, or absent altogether. An underactive thyroid can also cause your body to produce excess prolactin, a hormone normally involved in breastmilk production, which suppresses ovulation.
Thyroid disorders are relatively common in women and often develop gradually, so you might notice fatigue, weight changes, or sensitivity to temperature before connecting the dots to your cycle. A simple blood test can identify thyroid dysfunction, and treatment typically brings cycles back to normal.
Medications That Affect Your Period
Several types of medication can delay or stop periods by interfering with the hormones that drive your cycle. Some of the most common culprits include:
- Hormonal birth control: Certain forms, especially hormonal IUDs, implants, and continuous-use pills, can lighten periods significantly or stop them. After stopping birth control, it can take a few months for regular cycles to return.
- Antipsychotics and some antidepressants: These can raise prolactin levels by blocking dopamine receptors in the brain, which suppresses ovulation. Both older and newer antipsychotics carry this risk, as do certain tricyclic antidepressants and some SSRIs.
- Anti-seizure medications: Drugs like valproate and carbamazepine can increase androgen levels, disrupting ovulation in a pattern similar to PCOS.
- Opioids: Long-term opioid use suppresses reproductive hormones and frequently causes missed periods.
- Anabolic steroids: These raise androgen levels dramatically, often stopping periods entirely.
If you’ve started a new medication and noticed your cycle changing, that connection is worth discussing with your prescriber. In many cases, an alternative medication or dose adjustment can resolve the issue.
Perimenopause
If you’re in your 40s and your periods are becoming less predictable, perimenopause is a likely explanation. This transitional phase typically begins in the mid-40s, though it can start as early as the mid-30s in some women. It lasts an average of eight to ten years before menopause, when periods stop permanently.
During perimenopause, estrogen levels decline and fluctuate unevenly, throwing off the balance with progesterone. The result is cycles that may be shorter or longer than usual, heavier or lighter, or skipped entirely for a month or two before returning. You might also notice hot flashes, sleep disruption, or mood changes. A consistently elevated level of follicle-stimulating hormone (FSH) on blood testing can help confirm that perimenopause is underway, though hormone levels can swing enough during this phase that a single test isn’t always conclusive.
Other Medical Causes
Less commonly, a late period can signal other underlying conditions. Elevated prolactin from a small, benign pituitary growth can shut down ovulation. Chronic illnesses like uncontrolled diabetes or celiac disease can disrupt cycles through systemic inflammation and nutritional deficiencies. Scarring inside the uterus from procedures like a D&C can physically prevent the uterine lining from building up and shedding normally.
Premature ovarian insufficiency, where the ovaries stop functioning normally before age 40, is another possibility. It affects roughly 1 in 100 women and causes irregular or absent periods along with symptoms similar to menopause.
How Long Is Too Long
A period that’s a few days late isn’t unusual, especially if you’ve been sick, stressed, or traveling. Clinical guidelines recommend evaluation when previously regular periods stop for three months, or when previously irregular periods stop for six months. If your cycles consistently fall outside the 21-to-35-day range, or if you notice sudden changes in flow, duration, or regularity that persist for several cycles, that pattern is worth investigating even before the three-month mark.