Missed periods have a wide range of causes, from pregnancy and stress to hormonal conditions and significant changes in weight or exercise. Clinically, a missed period becomes notable when you’ve gone without a period for three or more cycle lengths (if your cycles were previously regular) or six or more months (if your cycles were already irregular). The most common first step is ruling out pregnancy, but beyond that, several underlying factors can disrupt the hormonal chain reaction your body relies on to menstruate each month.
Pregnancy: The Most Common Cause
If you’re sexually active and your period is late, pregnancy is the first thing to rule out. Home pregnancy tests detect a hormone called hCG in your urine, and they’re generally reliable by the day your period was expected. HCG can show up in urine as early as 10 days after conception, but testing before your missed period increases the chance of a false negative. A missed period typically falls around 14 days after conception, so waiting until that point gives the most accurate result.
Stress and Your Brain’s Hormonal Switch
Your menstrual cycle is controlled by a signaling chain that starts in the brain. A region of the brain sends out a pulsing hormonal signal that tells the ovaries when to release an egg. When you’re under significant stress, whether emotional, psychological, or physical, your body’s stress response system ramps up cortisol production. That elevated cortisol directly interferes with the brain’s reproductive signaling, slowing or stopping ovulation altogether.
This condition is called functional hypothalamic amenorrhea, and it doesn’t require extreme circumstances to develop. Externally imposed stressors like job loss, grief, or relationship problems can trigger it, but so can internalized stress patterns, such as perfectionism or anxiety about everyday situations. The key mechanism is that your body interprets ongoing stress as a signal to redirect energy toward survival systems, effectively deprioritizing reproduction. Periods typically return once the stressor resolves or stress management improves, though recovery can take weeks to months.
Low Energy Availability and Exercise
Your body needs a certain amount of energy to maintain all its systems. When you’re burning more calories through exercise than you’re taking in through food, or simply not eating enough, the resulting energy deficit triggers the same stress response that shuts down reproductive signaling. This is sometimes called Relative Energy Deficiency in Sport (RED-S), though it affects anyone in an energy deficit, not just athletes.
The process works through multiple pathways simultaneously. Low body fat reduces levels of leptin, a hormone that fat cells produce to signal energy status to the brain. Without adequate leptin, the brain reduces its reproductive hormone output. At the same time, the energy deficit raises cortisol, which further suppresses ovulation. The result is estrogen deficiency, which can cause symptoms beyond missed periods: breast tissue changes, vaginal dryness, and over time, loss of bone density. This isn’t limited to people who look underweight. Someone at a seemingly normal weight can still be in an energy deficit if their activity level outpaces their intake.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal disorders in people of reproductive age, and irregular or missed periods are a hallmark feature. It’s diagnosed when at least two of three criteria are present: elevated androgen levels (male-type hormones, which can cause acne or excess hair growth), irregular ovulation, and a characteristic appearance of the ovaries on ultrasound. Many people with PCOS have cycles longer than 35 days apart or fewer than eight cycles per year.
The underlying cause of PCOS is complex and not fully understood. What’s clear is that higher-than-normal androgen levels disrupt the normal ovulation process, meaning the ovary doesn’t release an egg on schedule. Without ovulation, the uterine lining doesn’t shed in the usual way, leading to skipped or very delayed periods. Some people with PCOS still have what look like regular cycles but aren’t actually ovulating, which is why the condition can sometimes go undiagnosed until someone has difficulty conceiving.
Thyroid Disorders
Your thyroid gland, located at the base of your neck, produces hormones that influence nearly every system in your body, including reproduction. Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause menstrual irregularities, but hypothyroidism is the more common culprit for missed or infrequent periods.
In one clinical study of people with hypothyroidism, 55% of those with elevated thyroid-stimulating hormone (TSH) levels experienced infrequent periods. Higher TSH levels correlated with more severe menstrual disruption. Thyroid hormones regulate reproductive processes from puberty through menopause, so when they’re out of balance, the downstream effects on your cycle can be significant. The good news is that menstrual irregularities caused by thyroid dysfunction often resolve once thyroid levels are brought back to a normal range with treatment.
High Prolactin Levels
Prolactin is the hormone responsible for milk production after childbirth, which is why breastfeeding commonly suppresses periods. But prolactin levels can also become elevated outside of pregnancy and breastfeeding. The most frequent cause is a small, benign growth on the pituitary gland called a prolactinoma.
Elevated prolactin disrupts your cycle through several mechanisms at once. It reduces the pulsing signal from the brain that drives ovulation, blocks the ovaries’ ability to respond to reproductive hormones, and can increase androgen levels. The net effect is that the ovaries stop releasing eggs and estrogen production drops, leading to missed periods. Prolactinomas are typically manageable with medication that brings prolactin levels back down, and periods usually resume once levels normalize.
Perimenopause
If you’re in your 40s and your periods are becoming unpredictable, perimenopause is a likely explanation. This transitional phase before menopause can start as early as the mid-30s or as late as the 50s, though most people notice changes in their 40s. During perimenopause, the ovaries gradually produce less estrogen, and ovulation becomes inconsistent. You might skip a month, then have a normal cycle, then skip two months. Cycles may also become shorter or longer than usual, and flow can vary from lighter to heavier than what you’re accustomed to.
Perimenopause lasts an average of four to eight years before menopause (defined as 12 consecutive months without a period). During this time, irregular periods are expected and don’t necessarily signal a problem. That said, very heavy bleeding, bleeding between periods, or periods that come more frequently than every 21 days during perimenopause are worth discussing with a healthcare provider, as they can occasionally point to other causes.
Other Contributing Factors
Several additional factors can cause or contribute to missed periods:
- Hormonal contraceptives: Some birth control methods, particularly hormonal IUDs, implants, and continuous-use pills, are designed to reduce or eliminate periods. After stopping hormonal birth control, it can take several months for your natural cycle to resume.
- Rapid weight changes: Gaining or losing a significant amount of weight in a short period can temporarily disrupt hormone levels enough to delay or skip ovulation.
- Chronic illness: Conditions like celiac disease, uncontrolled diabetes, or inflammatory bowel disease can affect nutrient absorption or hormone regulation enough to impact your cycle.
- Medications: Certain antipsychotics, antidepressants, and blood pressure medications can raise prolactin levels or otherwise interfere with reproductive hormones.
A single missed period, especially during a stressful month or after travel, is rarely a sign of something serious. But if you’ve missed three or more consecutive periods and pregnancy isn’t the cause, that pattern typically warrants a medical evaluation. Initial testing is straightforward: a pregnancy test, blood work to check thyroid function, prolactin, and reproductive hormone levels, and sometimes an ultrasound. Identifying the underlying cause is usually the fastest path to getting your cycle back on track.