Abnormal periods have a wide range of causes, from hormonal imbalances and structural changes in the uterus to stress, medications, and underlying health conditions. A period is generally considered abnormal if cycles are shorter than 21 days or longer than 35 days, if bleeding lasts more than 7 days, if you soak through a pad or tampon every hour, or if you regularly spot between periods. Understanding the specific cause matters because the path forward looks very different depending on what’s driving the change.
Hormonal Imbalances and Ovulation Problems
The most common reason periods become irregular is that ovulation isn’t happening on schedule, or isn’t happening at all. Your menstrual cycle depends on a carefully timed sequence of hormonal signals. When something disrupts that sequence, the uterine lining may build up unevenly, shed unpredictably, or not shed for weeks or months at a time.
Polycystic ovary syndrome (PCOS) is one of the leading culprits. Ovulation disorders are diagnosed in 75 to 85 percent of people with PCOS, and only about 30 percent ovulate on a regular schedule. The core problem involves insulin resistance: elevated insulin levels cause the ovaries to produce excess androgens (male-type hormones), which interfere with the hormonal signals that trigger egg release. Without ovulation, progesterone never rises, and the uterine lining keeps thickening instead of shedding in a predictable cycle. The result can be missed periods followed by unusually heavy bleeding when the lining finally breaks down.
Thyroid dysfunction works through a different pathway but produces similar results. An underactive thyroid disrupts the pulsing release of luteinizing hormone, which is the signal that triggers ovulation. Hypothyroidism can also raise levels of prolactin (a hormone normally elevated during breastfeeding), further suppressing ovulation. The downstream effects include irregular or heavy bleeding, spotting between periods, and in some cases, months without a period at all.
Structural Changes in the Uterus
Sometimes the problem isn’t hormonal but physical. Growths or tissue changes inside the uterus can alter how the lining builds up and sheds, leading to heavier or longer periods, bleeding between cycles, or both.
Fibroids are noncancerous growths in the muscular wall of the uterus. They’re extremely common, particularly in the 30s and 40s, and their effect on bleeding depends largely on location. Fibroids that press into the uterine cavity distort the lining and increase the surface area that bleeds each month. Polyps, which are smaller growths that project from the lining itself, can cause spotting between periods or heavier flow during them.
Adenomyosis is a condition where cells from the uterine lining grow into the muscular wall of the uterus. The mechanism is remarkably aggressive at a cellular level: endometrial cells gain an invasive quality that lets them migrate through the boundary between the lining and the muscle. This invasion creates a cycle of tissue damage and attempted repair. The injured tissue produces inflammatory compounds and local estrogen, which promote further growth while also triggering uterine contractions that cause more damage. The practical result is progressively heavier, more painful periods, often with significant cramping that starts days before bleeding begins.
Perimenopause and Age-Related Changes
If your periods have started behaving unpredictably in your 40s, perimenopause is a likely explanation. This transition phase typically begins in the mid-40s but can start as early as the 30s or as late as the 50s. During perimenopause, estrogen and progesterone levels rise and fall erratically rather than following their usual monthly pattern.
As ovulation becomes less reliable, cycles may stretch longer or compress shorter, flow may swing from barely-there to unusually heavy, and you may skip periods entirely. A useful benchmark: if the length of your cycle consistently shifts by seven days or more from what’s normal for you, that’s a sign of early perimenopause. If you go 60 days or more between periods, you’re likely in the later stage. These changes can last anywhere from a few years to a decade before periods stop completely.
Stress, Weight Changes, and Exercise
Your brain’s hypothalamus acts as a control center for your menstrual cycle, and it’s highly sensitive to signals about energy availability and stress. When your body perceives that resources are scarce or that you’re under sustained pressure, it dials down reproductive function to conserve energy for more immediate needs.
The mechanism is direct: stress activates the body’s cortisol-producing system, and elevated cortisol suppresses the pulsing hormonal signal that drives ovulation. This isn’t limited to emotional stress. Intense exercise, rapid weight loss, or chronic undereating all communicate the same message to the hypothalamus. Athletes with amenorrhea (absent periods) show both elevated cortisol and a downshift in thyroid function, a combination that minimizes energy expenditure while redirecting resources away from reproduction. The hunger hormone ghrelin also plays a role: when energy intake is too low, rising ghrelin levels directly slow the hormonal pulses needed for ovulation while simultaneously raising cortisol even further.
This type of period disruption, sometimes called hypothalamic amenorrhea, is reversible. Restoring adequate caloric intake and reducing physical or psychological stress typically allows normal cycles to resume, though it can take several months.
Bleeding Disorders
Heavy periods that have been present since your very first cycle may point to an inherited bleeding disorder rather than a gynecological problem. Von Willebrand disease, the most common inherited bleeding disorder, affects the blood’s ability to clot properly. Among people with chronically heavy menstrual bleeding, somewhere between 5 and 24 percent turn out to have von Willebrand disease. The prevalence is notably higher in white women (about 16 percent) compared to Black women (about 1 percent).
For context, clinically heavy bleeding means regularly losing more than 80 milliliters (about 2.7 ounces) of blood per period. In practical terms, that looks like needing to change pads or tampons every one to two hours, or emptying a menstrual cup after just a few hours. If that pattern sounds familiar and you also bruise easily, bleed heavily after dental work, or have a family history of bleeding problems, a blood test can identify the issue.
Infections and Pelvic Inflammation
Pelvic inflammatory disease (PID) develops when bacteria, most often from untreated gonorrhea or chlamydia, spread from the vagina into the uterus, fallopian tubes, or ovaries. Normally the cervix acts as a barrier against upward bacterial migration, but infection can compromise that defense.
PID can cause spotting or cramping throughout the month, bleeding between periods, unusual vaginal discharge, and pelvic pain. These symptoms sometimes appear gradually, which means the infection can simmer for weeks before someone seeks care. Left untreated, PID can cause scarring in the reproductive tract.
Medications and Contraceptives
Several medications directly change menstrual bleeding patterns, and this is one of the more overlooked causes of period changes.
- Copper IUD: Bleeding may stay the same or become heavier after placement. Unlike hormonal options, the copper IUD doesn’t thin the uterine lining, so some people experience noticeably increased flow.
- Hormonal IUD: The higher-dose hormonal IUD reduces bleeding by 71 to 91 percent. Many people eventually have very light periods or none at all.
- Blood thinners: Anticoagulant medications (used for blood clots, heart conditions, or stroke prevention) are associated with heavier menstrual bleeding. Studies show higher rates of significant bleeding at both 30 days and 6 months of use.
- Combined hormonal contraceptives: Birth control pills, patches, and rings that contain both estrogen and a progestin generally reduce menstrual bleeding. Breakthrough spotting is common in the first few months.
- Progestin-only methods: The injectable form often reduces bleeding over time, but progestin-only pills and the implant have less predictable bleeding patterns, with irregular spotting being common.
If your periods changed after starting a new medication, that connection is worth flagging with your provider. In many cases the bleeding patterns stabilize after a few months, but sometimes a different option is a better fit.
How Abnormal Bleeding Is Evaluated
Figuring out the cause usually starts with a detailed history of your cycle patterns and a physical exam. From there, the most common next steps include blood tests to check hormone levels, thyroid function, and clotting ability, along with an ultrasound to look at the uterus and ovaries. If the ultrasound suggests something inside the uterine cavity, a sonohysterography (where a small amount of fluid is placed in the uterus during the ultrasound) gives a clearer picture of polyps, fibroids, or other growths.
An endometrial biopsy, where a small sample of the uterine lining is taken and examined under a microscope, is sometimes recommended, particularly for people over 35 with persistent heavy or irregular bleeding. This checks for abnormal cell changes. MRI or CT scans are less commonly used but can help when other imaging isn’t conclusive, especially for conditions like adenomyosis that can be harder to see on standard ultrasound.