A period is considered irregular when the gap between cycles varies by more than seven to nine days from month to month, when cycles consistently fall outside the 21-to-35-day window, or when periods stop for three months or more. An estimated 14% to 25% of people of childbearing age experience menstrual irregularities at some point, and the causes range from everyday stress to underlying medical conditions.
How a Regular Cycle Works
Your menstrual cycle is controlled by a chain of hormone signals that runs from your brain to your ovaries and back. The hypothalamus, a small region deep in the brain, releases a hormone that tells the pituitary gland to produce two key signals: one that triggers egg development in the ovaries, and another that triggers ovulation. As an egg-containing follicle grows, it releases estrogen, which feeds back to the pituitary and eventually causes a surge that releases the egg. After ovulation, the empty follicle produces progesterone, which prepares the uterine lining for a potential pregnancy. If pregnancy doesn’t happen, progesterone drops, and the lining sheds as your period.
This feedback loop is precise. Anything that disrupts the timing or strength of these signals, whether it originates in the brain, the thyroid, the ovaries, or elsewhere, can delay or prevent ovulation and throw off your cycle.
Stress and Cortisol
Chronic stress is one of the most common and underappreciated causes of irregular periods. When you’re under prolonged psychological or physical stress, your body produces elevated levels of cortisol. Cortisol interferes with your cycle in two ways: it reduces the pituitary gland’s sensitivity to the brain’s reproductive signals, and it suppresses the release of those signals from the hypothalamus in the first place. The result is weaker or less frequent hormonal pulses, which can delay ovulation or prevent it entirely.
Researchers have identified the specific brain cells cortisol acts on. It appears to shift the balance of signaling chemicals in a cluster of neurons that serve as the brain’s reproductive pacemaker, increasing inhibitory signals and decreasing stimulatory ones. This doesn’t just slow ovulation. It can also suppress sexual behavior and, in severe cases, stop periods altogether. The effect is reversible once the source of stress resolves or is managed, but ongoing stress can keep cycles unpredictable for months.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal conditions behind irregular periods. It’s characterized by infrequent ovulation and higher-than-normal levels of androgens (hormones like testosterone that are present in everyone but elevated in PCOS). People with PCOS typically have fewer than nine periods a year, and some go three or more consecutive months without one.
The underlying problem in PCOS is a disrupted feedback loop. The ovaries produce excess androgens, which interfere with the normal maturation and release of eggs. Follicles may start developing but stall partway, leading to the characteristic “polycystic” appearance on an ultrasound. Without ovulation, progesterone never rises, the uterine lining keeps building, and periods become heavy, unpredictable, or absent.
Thyroid Disorders
Both an underactive and an overactive thyroid can make periods irregular, though the mechanisms differ. An underactive thyroid (hypothyroidism) triggers a chain reaction: the brain compensates by increasing production of a hormone called TRH, which also stimulates prolactin release. Elevated prolactin directly blocks the brain’s reproductive signaling, suppressing ovulation. Hypothyroidism also reduces the number of progesterone receptors in reproductive tissue while increasing estrogen receptors, further disrupting the hormonal balance needed for regular cycles.
On top of that, thyroid hormones normally help ovarian cells respond to the signals that drive egg development. When thyroid function is off in either direction, the ovaries become less responsive, and cycles can shorten, lengthen, or stop. The menstrual changes often resolve once thyroid levels are corrected, which is why thyroid testing is a standard part of evaluating irregular periods.
Weight, Nutrition, and Energy Availability
Your body needs a certain amount of available energy to sustain a menstrual cycle. When calorie intake drops too low relative to what you burn, especially in athletes or people with restrictive eating patterns, the brain slows or shuts down reproductive hormone signals. This condition, known as relative energy deficiency in sport (REDs), causes irregular periods or complete loss of periods. Low body fat percentage, poor nutrition, and high physical demands all contribute. Recovery typically requires increasing daily energy intake, with guidelines suggesting an increase of roughly 300 to 450 calories per kilogram of lean body mass per day.
On the other end of the spectrum, excess body weight can also disrupt cycles. Fat tissue produces estrogen, and higher levels of circulating estrogen can interfere with the precise hormonal feedback loop that triggers ovulation. This is one reason PCOS and excess weight frequently overlap, though each can occur independently.
Medications That Disrupt Cycles
Several common medication classes can cause periods to become irregular or stop entirely, even when that’s not their intended purpose. The main culprits include:
- Antipsychotics such as risperidone, olanzapine, and haloperidol, which raise prolactin levels and suppress reproductive hormone signaling
- Antidepressants including SSRIs and tricyclics, which can also increase prolactin
- Opioid pain medications like codeine and morphine, which suppress the brain’s reproductive signals
- Anti-seizure drugs such as valproate and carbamazepine
- Anabolic steroids and testosterone supplements, which shift the balance of sex hormones away from the pattern needed for ovulation
Many of these drugs work through the same mechanism: raising prolactin. Prolactin acts directly on the neurons that produce reproductive hormones, blocking their release. If your periods changed after starting a new medication, that connection is worth raising with whoever prescribed it.
Life Stage Transitions
Irregular periods are normal at the beginning and end of your reproductive years. In the first few years after a first period, cycles commonly range from 21 to 45 days because ovulation hasn’t yet settled into a predictable rhythm. By the third year, 60% to 80% of cycles fall into the adult range of 21 to 34 days, but it can take longer for some people.
At the other end, perimenopause brings a gradual decline in ovarian function that makes cycles increasingly unpredictable. In early perimenopause, cycle length may vary by seven or more days from what’s been normal for you. In late perimenopause, gaps of 60 days or more between periods are common. These changes can begin in your early 40s or even late 30s and last for several years before periods stop entirely. The underlying cause is the same one that drives irregularity in adolescence, just in reverse: the ovaries are becoming less responsive to signals from the brain, and ovulation becomes sporadic.
Other Medical Causes
Several other conditions can make periods irregular. Elevated prolactin from a small benign pituitary growth (called a prolactinoma) blocks reproductive hormones the same way stress or medications do. Premature ovarian insufficiency, where the ovaries stop functioning normally before age 40, causes cycles to space out and eventually stop. Uterine fibroids or polyps don’t typically change cycle timing but can alter flow and duration, which many people experience as irregularity. Uncontrolled diabetes, celiac disease, and other chronic illnesses can also interfere with the hormonal chain of command.
When Irregular Becomes Concerning
Some irregularity is expected during adolescence and perimenopause. Outside those windows, certain patterns warrant a medical evaluation. If you previously had regular cycles and go more than three months without a period, that meets the clinical definition of secondary amenorrhea and should be investigated. If your cycles have always been irregular and you go six months without bleeding, the same applies. Consistently short cycles (under 21 days), very long cycles (over 35 days in adults), or bleeding that lasts more than seven days are also worth discussing.
Evaluation typically involves blood tests to check thyroid function, prolactin, and reproductive hormone levels, along with an ultrasound of the ovaries if PCOS or structural issues are suspected. Most causes of irregular periods are treatable once identified.