Irregular periods affect an estimated 14% to 25% of women of childbearing age. A normal menstrual cycle ranges from 21 to 35 days, and your period is considered irregular if cycles consistently fall outside that window, if the length between cycles varies by more than nine days, or if you skip periods entirely. The causes range from hormonal imbalances and thyroid problems to lifestyle factors and natural life transitions.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal causes of irregular periods. It’s diagnosed when at least two of three features are present: elevated androgen levels (the hormones often called “male hormones,” though all women produce them), irregular or absent ovulation, and cysts visible on the ovaries via ultrasound. Many women with PCOS go months between periods or have unpredictable cycle lengths because their ovaries don’t release an egg on a regular schedule.
The hormonal picture in PCOS often involves higher-than-normal testosterone levels. Doctors typically measure both total and free testosterone to check for this. You might also notice symptoms driven by those elevated androgens: acne along the jawline, thinning hair on the scalp, or excess hair growth on the face and body. Not everyone with PCOS has all of these signs, which is part of why it can take time to diagnose.
Thyroid Problems
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can disrupt your cycle. The connection runs through several pathways. Thyroid hormones directly influence the ovaries, and abnormal thyroid function can alter levels of prolactin and other reproductive hormones. In hypothyroidism specifically, the brain releases more of a signaling hormone called TRH, which can raise prolactin levels. Elevated prolactin suppresses ovulation, leading to missed or very late periods.
Thyroid-related menstrual changes can look like infrequent periods, skipped periods, or unusually heavy bleeding. Women with hypothyroidism also face a higher risk of pregnancy loss. The good news is that once thyroid levels are corrected with medication, menstrual cycles often return to normal relatively quickly.
Stress and Energy Deficits
Your brain actively monitors your body’s energy balance, and when it senses a deficit, it can shut down reproductive function to conserve resources. This happens through a process where the hypothalamus, a small region at the base of the brain that orchestrates your hormonal cycles, slows or stops sending the signals that trigger ovulation.
Research on healthy women ages 18 to 30 has shown that the key trigger isn’t exercise alone or dieting alone, but overall energy availability: the calories you take in minus the calories you burn during exercise. When that number drops below about 30 calories per kilogram of lean body mass per day, the brain begins reducing its reproductive signaling. For a woman with 45 kg of lean body mass, that threshold is roughly 1,350 calories of available energy after accounting for exercise.
This explains why irregular periods are common in endurance athletes, people with eating disorders, and anyone combining heavy training with restrictive eating. Chronic psychological stress can trigger the same shutdown through a different route, raising cortisol levels that interfere with the same hormonal cascade. In both cases, the body is essentially deciding that conditions aren’t favorable for pregnancy and pausing the cycle accordingly.
Perimenopause
The transition to menopause, called perimenopause, typically begins in a woman’s 40s but can start in the late 30s. It’s one of the most common and most overlooked causes of irregular periods, partly because many women don’t expect cycle changes until closer to 50.
The changes happen in stages. Early on, cycles tend to get slightly shorter than usual. As the transition progresses, cycle length starts to vary by seven days or more from one month to the next. In late perimenopause, it’s common to skip 60 days or more between periods. The whole process can last anywhere from a few years to a decade before periods stop entirely. Fluctuating estrogen and progesterone levels drive these changes, and they can also bring heavier or lighter flow, hot flashes, and sleep disruption.
Uterine Polyps and Fibroids
Structural growths in the uterus can cause bleeding patterns that mimic or overlap with irregular periods. Uterine polyps are soft tissue growths that attach to the inner wall of the uterus and are sensitive to estrogen, meaning they grow in response to your body’s natural estrogen production. They can cause bleeding between periods, unpredictable periods that vary in length and heaviness, and very heavy flow.
Fibroids, which are noncancerous growths in the muscular wall of the uterus, produce similar symptoms. Both conditions are common, particularly in women over 30, and both tend to cause changes in bleeding patterns rather than changes in cycle timing. If your periods come on schedule but involve spotting between cycles or dramatically heavier flow, a structural cause is worth investigating.
Endometriosis
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus. It’s primarily known for causing severe pain, but it also affects bleeding patterns. Women with endometriosis may experience heavy menstrual periods, bleeding between periods, and cycles that are shorter than average (under 27 days). Periods that last longer than seven days are also more common.
The relationship between endometriosis and cycle irregularity is somewhat bidirectional. Shorter, heavier cycles increase menstrual flow into the pelvic cavity, which may contribute to the development of endometriosis in the first place. At the same time, the inflammation caused by endometriosis can further disrupt normal bleeding patterns.
Other Common Causes
Several other factors can throw off your cycle. Significant weight changes in either direction alter estrogen production, since fat tissue is a major source of estrogen. Rapid weight loss can suppress ovulation, while significant weight gain can lead to excess estrogen that disrupts the normal hormonal rhythm.
Prolactin-producing tumors of the pituitary gland (usually small and benign) can suppress ovulation in the same way that thyroid-driven prolactin elevation does. Certain medications, including some antipsychotics and anti-nausea drugs, can raise prolactin levels as a side effect.
Breastfeeding commonly delays the return of regular periods after childbirth, sometimes for months. This is a normal physiological response, not a medical concern. Travel across time zones, illness, and disrupted sleep patterns can also cause a one-off late or early period without signaling any underlying problem. The distinction that matters is between an occasional off cycle and a persistent pattern of irregularity lasting three months or more.