A late period can be caused by pregnancy, stress, weight changes, hormonal conditions, certain medications, or simply normal variation in your cycle. A healthy menstrual cycle ranges from 21 to 35 days, and occasional shifts of a few days in either direction are common. Your period is generally considered late if it hasn’t arrived within a week of when you expected it, and clinically missed if it’s absent for three or more months in a row.
What Counts as a Late Period
Many people think of their cycle as a fixed 28-day clock, but that number is just an average. Normal cycles fall anywhere between 21 and 35 days, and the same person’s cycle can vary by several days from month to month. A period that shows up on day 32 one month and day 27 the next is still within normal range.
A period becomes worth paying attention to when it’s more than a week past your usual pattern, especially if you’ve been tracking consistently. If your cycles were previously regular and your period has been absent for more than three months, that meets the clinical definition of secondary amenorrhea and warrants a medical workup. For people whose cycles have always been irregular, the threshold extends to six months.
Pregnancy
The most obvious explanation for a missed period is pregnancy, and it’s the first thing to rule out. If you’re sexually active and your period is even a week late, a home pregnancy test can give you a reliable answer. These tests detect a hormone called hCG in your urine, which becomes detectable about 12 to 15 days after ovulation in a typical 28-day cycle. Testing too early can produce a false negative, so if your first test is negative but your period still doesn’t come, retesting a few days later improves accuracy.
Stress and Sleep Disruption
Your brain controls the hormonal signals that trigger ovulation. When you’re under significant stress, whether physical or emotional, those signals can slow down or pause entirely. This doesn’t require a dramatic life event. A stretch of poor sleep, a demanding work deadline, travel across time zones, or even worrying about a late period itself can be enough to delay ovulation by days or weeks. Since your period arrives a fixed number of days after ovulation (typically around 14), pushing ovulation later automatically pushes your period later too.
This kind of delay usually resolves on its own once the stressor passes. But chronic, ongoing stress can lead to repeatedly irregular or skipped cycles because the brain keeps dialing down reproductive hormone output to conserve energy for what it perceives as survival priorities.
Weight and Body Composition
Both ends of the weight spectrum can disrupt your cycle. Being significantly underweight or having very low body fat suppresses the hormonal pulses your brain sends to trigger ovulation. This is especially common in people with restrictive eating patterns. Research has shown that calorie deprivation is the actual driver here, not exercise alone. Studies that separated the effects found that exercise without energy deprivation did not disrupt hormonal cycles, while calorie restriction did, even without intense physical activity.
On the other end, higher body weight is associated with longer and more variable cycles. Fat tissue produces small amounts of estrogen on top of what the ovaries make. In people with excess fat tissue, this extra estrogen can suppress normal ovarian function and interrupt the regular hormonal rhythm that drives your cycle. A large Harvard study found that people with a BMI of 30 or above tended to have notably longer and less predictable cycles compared to those in lower BMI ranges.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal conditions in people of reproductive age, and irregular or late periods are its hallmark symptom. In PCOS, the ovaries produce higher than normal levels of androgens (often called “male hormones,” though everyone produces them). This hormonal imbalance can prevent eggs from maturing and releasing on a regular schedule, which means ovulation happens late, infrequently, or not at all.
Other signs of PCOS include acne, excess hair growth on the face or body, thinning hair on the scalp, and difficulty losing weight. Diagnosis typically requires two of three features: irregular cycles, elevated androgen levels (shown by blood work or physical symptoms), and a characteristic appearance of the ovaries on ultrasound or elevated levels of anti-Müllerian hormone on a blood test. If your periods are consistently unpredictable and you notice any of these other signs, PCOS is worth discussing with a provider.
Thyroid Problems
Your thyroid gland sets the pace for many processes in your body, including your menstrual cycle. An underactive thyroid (hypothyroidism) can delay or stop periods through a chain reaction: low thyroid hormone triggers your brain to release more of the signal that stimulates the thyroid, and that same signal also increases production of prolactin, a hormone normally involved in breastfeeding. Elevated prolactin suppresses the hormones needed for ovulation.
An overactive thyroid (hyperthyroidism) can also cause irregular cycles, though the mechanism differs. Either way, thyroid disorders are diagnosed with a simple blood test and are highly treatable. If your late periods come alongside fatigue, unexplained weight changes, feeling unusually cold or warm, or changes in your hair and skin, thyroid function is a reasonable thing to check.
Medications That Delay Periods
Several categories of medication can cause late or absent periods, often by raising prolactin levels or shifting your hormonal balance.
- Antipsychotics are among the most common culprits, as many directly increase prolactin production.
- Antidepressants, including SSRIs and tricyclics, can have a similar effect in some people.
- Hormonal contraceptives intentionally alter your cycle. After stopping birth control pills, implants, or hormonal IUDs, it can take several months for your natural cycle to resume.
- Opioid pain medications can suppress reproductive hormones with regular use.
- Anti-seizure medications can shift the balance between estrogen and androgens.
- Blood pressure medications like methyldopa and verapamil may raise prolactin.
If you started a new medication and your periods became irregular, that connection is worth raising with whoever prescribed it. Stopping medication on your own to “fix” your cycle is not the answer, but alternatives or dosage adjustments may exist.
Perimenopause
If you’re in your 40s and your periods are becoming unpredictable, perimenopause is a likely explanation. This transitional phase typically begins in the mid-40s, though it can start as early as the mid-30s. It lasts an average of eight to ten years before periods stop entirely (menopause).
During perimenopause, estrogen and progesterone levels fluctuate erratically rather than declining in a straight line. This means your cycles might get shorter for a while, then longer, then skip entirely for a month or two before returning. You might also notice heavier or lighter flow, hot flashes, sleep changes, or mood shifts. Hormone blood tests during this phase can be misleading because levels bounce around so much from week to week.
Other Common Triggers
A few other situations commonly cause a one-off late period without indicating a larger problem. Illness, particularly anything that causes a fever, can delay ovulation for that cycle. Rapid weight loss from a crash diet can temporarily suppress your cycle even if your overall weight remains in a healthy range. Jet lag and shift work disrupt circadian rhythms, which are closely tied to hormonal timing. And for the first couple of years after getting your first period, irregular cycles are normal as the reproductive system matures.
A single late period, especially during a stressful month or after a lifestyle change, rarely signals something serious. The pattern matters more than any individual cycle. Consistently irregular periods, cycles that regularly exceed 35 days, or periods that disappear for three months or more point toward something worth investigating with blood work or imaging.