A normal menstrual cycle falls between 21 and 35 days, and a variation of more than 20 days from one cycle to the next is considered irregular. If your period is always unpredictable, the cause is almost certainly one of a handful of common conditions, most of them treatable. Understanding which one applies to you starts with recognizing the patterns your body is showing alongside the irregularity.
PCOS Is the Most Common Cause
Polycystic ovary syndrome affects how your ovaries release eggs and is the single most frequent reason for persistently irregular periods in people of reproductive age. It’s diagnosed when you have at least two of three features: higher-than-normal levels of androgens (sometimes visible as acne, thinning hair, or excess facial and body hair), ovulation that happens inconsistently or not at all, and ovaries that contain a large number of small follicles on ultrasound.
The hormonal picture in PCOS creates a feedback loop. Elevated androgens interfere with the normal monthly process of maturing and releasing an egg. Without ovulation, your uterine lining doesn’t get the hormonal signal to shed on schedule, so periods come late, skip entirely, or arrive unpredictably. Some people with PCOS go months without a period, while others have cycles that swing between 25 and 60 days with no discernible pattern.
Vitamin D status appears to play a role in how severe those ovulatory problems become. In one large study of infertile women with PCOS, over 70% had vitamin D levels below 20 ng/ml, and over 90% were below 30 ng/ml. Women with PCOS who are also vitamin D deficient ovulate at significantly lower rates. The mechanism involves disrupted follicle development and increased inflammation in ovarian tissue. Supplementing vitamin D may partially reverse irregular follicle development, though it isn’t a standalone fix for PCOS.
Your Thyroid Controls More Than You Think
Thyroid hormones are deeply involved in menstrual regulation, and even mild thyroid dysfunction can throw off your cycle. Your thyroid hormones (T3 and T4) influence your reproductive system at nearly every level: they regulate the brain signals that trigger ovulation, they help ovarian cells respond to the hormones that mature eggs, and they affect how your body transports estrogen and progesterone through the bloodstream.
When your thyroid is underactive, the chain reaction starts at the top. Reduced thyroid hormone leads to lower production of kisspeptin, a brain chemical that kick-starts the cascade ending in ovulation. At the same time, an underactive thyroid can raise prolactin levels, which further suppresses ovulation. T3 normally keeps prolactin in check in a dose-dependent way, so when T3 drops, prolactin rises, and periods become irregular or stop.
At the ovary itself, thyroid hormones help granulosa cells (the cells that nurture a developing egg) survive and multiply. They also boost the number of receptors those cells have for follicle-stimulating hormone, which is essential for egg maturation. Without adequate thyroid function, eggs don’t develop properly, ovulation is unreliable, and cycles become unpredictable. The good news is that treating the thyroid problem typically restores regular cycles, often within a few months.
Your Age May Be the Explanation
If you’re in your teens, irregular periods are expected. The menstrual cycle in adolescents can range from 21 to 45 days, and it often takes several years after your first period for cycles to settle into a consistent rhythm. The hormonal feedback system between the brain and ovaries is still calibrating, and occasional skipped or unusually long cycles during this window aren’t a sign of a problem.
On the other end, perimenopause is a common and frequently overlooked explanation. Most people notice changes in their 40s, but some experience perimenopausal shifts as early as their 30s. Cycles may become shorter, then longer, then unpredictable. Periods might be heavier one month and barely there the next. This transition phase can last years before periods stop entirely. If you’re in your late 30s or 40s and your previously regular cycle has become erratic, perimenopause is a likely explanation.
Birth Control Can Cause Irregularity, Not Just Prevent It
Hormonal contraceptives are sometimes prescribed to regulate periods, but they can also be the source of irregular bleeding, especially in the early months. With hormonal IUDs, spotting and irregular bleeding in the first 2 to 6 months is common and usually resolves on its own. With the implant, the bleeding pattern you experience in the first 3 months tends to be the pattern you’ll have going forward, so persistent irregularity with an implant is worth discussing with your provider.
Low-dose and ultra-low-dose birth control pills are more likely to cause breakthrough bleeding than standard-dose formulations. Smoking increases the risk, as does skipping pills or taking them at inconsistent times. People who use pills or the ring continuously to skip periods altogether also experience more breakthrough bleeding. And stopping hormonal birth control can cause temporary irregularity as your body’s own hormonal cycle restarts, sometimes taking several months to find its rhythm again.
Emergency contraception pills can also trigger off-schedule bleeding. If you’ve used one recently, an unusual cycle or two afterward is expected.
Other Medical Causes Worth Knowing
Uterine fibroids, which are benign growths in the uterine wall, can cause heavy or irregular bleeding that has nothing to do with ovulation or hormones. They’re extremely common, particularly in your 30s and 40s, and the bleeding they cause is often heavier and longer rather than unpredictably timed.
Untreated infections like chlamydia or gonorrhea can also contribute to irregular bleeding. These infections may cause spotting between periods or bleeding after sex, and since they can be asymptomatic for a long time, they’re easy to miss without screening.
Significant changes in weight, extreme exercise, or prolonged stress can all suppress the hormonal signals needed for ovulation. Your body essentially deprioritizes reproduction when it perceives insufficient energy or high threat, leading to missed or delayed periods. This is more common than many people realize and is often reversible by addressing the underlying stressor.
When Irregular Periods Need Medical Attention
Not every irregular cycle needs investigation, but certain patterns do. You should schedule an evaluation if you’ve had no period at all for more than three months, or if you’ve had a consistently irregular or unpredictable pattern for three months or longer. Bleeding between periods that persists for more than three consecutive months also warrants a visit.
Heavy bleeding is its own category of concern. Soaking through a pad or tampon every hour for more than two hours straight, feeling lightheaded or dizzy from blood loss, or having periods that regularly last more than seven days are all signs that something beyond normal variation is happening.
Severe cramps that don’t respond to over-the-counter pain relievers, aren’t helped by heat or rest, and interfere with your ability to work or go about your day may point to conditions like endometriosis or adenomyosis, both of which can also affect cycle regularity. If something feels persistently off for your body, whether that’s the timing, the flow, the pain, or how your cycle affects your energy and mood, that’s reason enough to bring it up.