A normal menstrual cycle falls between 21 and 35 days, with bleeding lasting 2 to 7 days. If your cycles consistently fall outside that window, vary widely in length from month to month, or disappear entirely for stretches, your period counts as irregular. The causes range from everyday factors like stress and weight changes to underlying conditions that need treatment.
How Stress Disrupts Your Cycle
When you’re under sustained stress, your body produces more cortisol. Cortisol directly interferes with the hormonal chain reaction that triggers ovulation. Specifically, elevated cortisol reduces how often your brain sends out the pulses of luteinizing hormone that tell your ovaries to release an egg. It also makes the pituitary gland less responsive to the signals that normally keep the cycle on track.
This doesn’t require a major life crisis. Chronic work pressure, sleep deprivation, intense exercise routines, or significant calorie restriction can all keep cortisol high enough to delay or skip ovulation. When ovulation is delayed, your period arrives late. When it’s skipped entirely, you miss a period altogether. The cycle typically corrects itself once the stressor resolves, but prolonged disruption can mean months of irregularity.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common hormonal causes of irregular periods, affecting roughly 1 in 10 women of reproductive age. The hallmark is an excess of androgens, often called “male hormones,” though all women produce them in smaller amounts. Elevated testosterone or related hormones disrupt the normal ovulation cycle, leading to long gaps between periods or skipped periods entirely.
Excess body hair, particularly on the face, chest, or back, is one of the strongest outward signs pointing toward PCOS. Acne and hair thinning on the scalp can also appear, though they’re less reliable indicators on their own. Diagnosis involves blood tests measuring testosterone levels and sometimes an ultrasound to check for characteristic changes in the ovaries. Not everyone with PCOS has visible cysts, and the name is somewhat misleading. The core issue is hormonal imbalance, not the ovaries themselves.
Weight management, if relevant, can improve symptoms because fat tissue produces estrogen, which further disrupts the hormonal balance. Hormonal contraceptives are commonly used to regulate the cycle, and other medications can target specific symptoms like excess hair growth or difficulty conceiving.
Thyroid Problems
Your thyroid gland, located at the front of your neck, produces hormones that influence nearly every system in your body, including reproduction. When the thyroid is underactive (hypothyroidism), the resulting hormone shifts trigger a chain reaction: the brain compensates by increasing production of a thyroid-stimulating signal, which inadvertently raises prolactin levels. Prolactin is the same hormone responsible for milk production after pregnancy, and at elevated levels it suppresses the hormonal signals that drive ovulation.
An overactive thyroid (hyperthyroidism) can also shorten or lighten periods. Either direction of thyroid dysfunction can make cycles unpredictable. The good news is that thyroid conditions are straightforward to detect with a simple blood test and typically respond well to treatment. Once thyroid hormone levels normalize, menstrual regularity usually returns within a few months.
Perimenopause
If you’re in your 40s and your previously clockwork cycle has become unpredictable, perimenopause is the most likely explanation. This transitional phase begins about 8 to 10 years before menopause, typically starting in the mid-40s, though some women notice changes as early as their mid-30s.
During perimenopause, estrogen levels don’t decline in a smooth, gradual line. They fluctuate dramatically, sometimes spiking higher than they did during your peak reproductive years before dropping sharply. This hormonal rollercoaster throws off the balance with progesterone, the other key reproductive hormone. The result is cycles that may suddenly be shorter, then longer, then absent for a month or two before returning. Flow can change too, swinging between unusually heavy and unusually light. These fluctuations are a normal part of the transition, though heavy or prolonged bleeding still warrants evaluation to rule out other causes.
Structural Growths in the Uterus
Fibroids and polyps are physical growths that can change your bleeding pattern without affecting your hormones. Uterine polyps are soft tissue growths attached to the inner wall of the uterus. They’re estrogen-sensitive, meaning they grow in response to your body’s natural estrogen. Polyps can cause bleeding between periods, unpredictable cycle timing, and unusually heavy flow. Some women with polyps have no symptoms at all and only discover them during unrelated imaging.
Fibroids are firmer growths made of muscle tissue. Depending on their size and location, they can cause heavy bleeding, longer periods, or spotting between cycles. The key distinction from hormonal causes is that structural growths tend to affect bleeding volume and timing without necessarily changing your overall cycle length. You might still ovulate on schedule but bleed heavily or spot at unexpected times. Both conditions are typically identified through ultrasound and are treatable.
Hormonal Contraceptives
If you’ve recently started, stopped, or switched birth control, irregular bleeding is common and often temporary. Progestin-only methods (the mini-pill, hormonal IUDs, implants, and injections) are particularly known for causing unpredictable spotting or breakthrough bleeding, especially in the first several months. The exact reason isn’t fully understood, but it involves changes to the uterine lining’s blood vessel stability and shifts in local hormone levels within the uterus.
Combined pills that contain both estrogen and progestin can also cause irregular spotting, especially if you miss doses or take them at inconsistent times. Stopping hormonal contraception after long-term use sometimes leads to a delay of several months before regular ovulation resumes. This is generally harmless but can be unsettling if you’re not expecting it.
Other Common Causes
Significant weight changes in either direction can disrupt your cycle. Very low body fat reduces estrogen production, which can stop periods entirely, something frequently seen in competitive athletes and people with restrictive eating patterns. Conversely, excess body fat increases estrogen production, which can lead to heavier, less predictable periods and raise the risk of conditions like PCOS.
Elevated prolactin from causes other than thyroid disease, such as certain medications or a small benign pituitary growth, can suppress ovulation and cause missed periods. Conditions affecting the adrenal glands, which sit above your kidneys and produce cortisol and small amounts of androgens, can mimic some of the hormonal patterns seen in PCOS.
The first few years after your first period are also commonly irregular. It takes time for the hormonal feedback system to mature, and cycles that vary by a week or more are typical for adolescents. Most young women settle into a more predictable pattern within two to three years.
What Doctors Test For
When you bring up irregular periods, the initial workup usually starts with blood tests. The standard panel checks thyroid-stimulating hormone (to evaluate thyroid function), prolactin, cortisol, follicle-stimulating hormone (FSH), and testosterone. If PCOS is suspected, additional androgen-related hormones may be measured, ideally using highly accurate lab methods since standard tests can miss mild elevations.
An ultrasound may be ordered to look for structural issues like polyps, fibroids, or ovarian changes. In some cases, a progesterone level drawn at a specific point in your cycle can confirm whether or not you’re ovulating.
Signs That Need Prompt Attention
Most irregular periods reflect manageable, non-dangerous causes. But certain patterns warrant faster evaluation: periods that suddenly become much heavier than your normal, bleeding that lasts longer than seven days, or cycles that consistently come fewer than 21 days apart. If you’re soaking through a pad or tampon every hour for more than two hours straight and also feel dizzy, lightheaded, or short of breath, that’s a reason to seek emergency care. Bleeding after menopause, even light spotting, always needs evaluation since it can signal changes to the uterine lining that require treatment.