Abnormal vaginal bleeding has a wide range of causes, from hormonal shifts and benign growths to infections and, less commonly, cancer. A normal menstrual cycle falls between 21 and 35 days, lasts seven days or fewer, and requires roughly three to six pads or tampons per day. Bleeding that falls outside those boundaries, happens between periods, or occurs after menopause counts as abnormal and has a specific, identifiable cause in most cases.
What Counts as Abnormal
Knowing the boundaries of a typical cycle helps you recognize when something is off. A period that arrives more often than every 21 days or less often than every 35 days is outside the normal range. Flow lasting longer than seven days, soaking through a pad or tampon every one to two hours, or requiring you to double up on products all qualify as excessively heavy. Spotting between periods, bleeding after sex, and any bleeding after menopause are also considered abnormal regardless of how light they seem.
Hormonal and Ovulatory Causes
The most common reason for irregular bleeding in people who menstruate is a disruption in ovulation. Normally, the ovaries release an egg each cycle, which triggers a shift in hormones that stabilizes the uterine lining and produces a predictable period. When ovulation doesn’t happen, the lining keeps thickening under the influence of estrogen without the counterbalancing hormone (progesterone) that would normally organize and shed it on schedule. The result is unpredictable breakthrough bleeding or unusually heavy periods when the lining finally does shed.
Polycystic ovary syndrome (PCOS) is one of the most frequent causes of this pattern. Thyroid disorders, significant weight changes, extreme stress, and the natural transitions of puberty and perimenopause can also disrupt ovulation. In adolescents, irregular cycles are common for the first few years after periods begin. By the third year, 60 to 80 percent of cycles settle into the typical 21-to-34-day adult range. Cycles that remain absent for more than 90 days at any age deserve evaluation.
Uterine Polyps and Fibroids
Polyps are small overgrowths of the uterine lining that develop when cells multiply more than they should. They’re estrogen-sensitive, meaning they grow in response to the body’s estrogen levels, and they’re most common in people approaching or past menopause. Polyps can cause bleeding between periods, unusually heavy flow, and postmenopausal bleeding.
Fibroids (also called leiomyomas) are noncancerous muscle growths in or on the uterine wall. They’re extremely common, especially in people over 30. Fibroids that grow into the uterine cavity or distort its shape tend to cause the heaviest bleeding because they increase the surface area of the lining and interfere with the uterus’s ability to contract and stop blood flow after a period. Both polyps and fibroids are structural problems, meaning they can often be seen on imaging and treated directly.
Infections and Inflammation
Pelvic inflammatory disease (PID) is an infection of the upper reproductive tract that can cause abnormal bleeding, pain during sex, and unusual discharge. It’s most often caused by sexually transmitted bacteria like chlamydia and gonorrhea, though other organisms can be involved. The infection inflames the uterine lining (endometritis), the fallopian tubes, or both. Some cases are subtle enough that the only sign is irregular spotting or light bleeding between periods, which is easy to dismiss. Cervical infections on their own can also cause the cervix to become fragile and bleed on contact, such as after intercourse or a pelvic exam.
Medications That Cause Bleeding
Several common medications alter bleeding patterns as a side effect. Blood thinners, particularly the newer oral anticoagulants like rivaroxaban, carry a well-documented risk of heavier menstrual bleeding. Anyone starting a blood thinner should be aware of this possibility before it catches them off guard.
Hormonal contraceptives can cause irregular spotting, especially in the first few months of use. This includes birth control pills, hormonal IUDs, implants, and injections. Copper IUDs, which contain no hormones, are known to make periods heavier and longer. Hormone replacement therapy used during menopause can also trigger unexpected bleeding, particularly if the dose or type needs adjustment. Stopping or inconsistently taking hormonal birth control is another frequent cause of breakthrough bleeding.
Adenomyosis
Adenomyosis happens when tissue from the uterine lining grows into the muscular wall of the uterus. This makes the uterus enlarge and can cause intensely painful, heavy periods. It’s different from endometriosis, where similar tissue grows outside the uterus. Adenomyosis is most common in people in their 30s and 40s and is often discovered alongside fibroids, which can make pinpointing the exact cause of heavy bleeding more complicated.
Endometrial Hyperplasia and Cancer
When the uterine lining thickens excessively without being shed regularly, the condition is called endometrial hyperplasia. Over time, some forms of hyperplasia can progress to endometrial cancer. The risk factors for both overlap significantly: higher body weight, diabetes, hypertension, and prolonged periods without ovulation. In a large study of women 45 and younger with abnormal bleeding, about 7 percent had hyperplasia and 1.6 percent had endometrial cancer. Women with cancer in that study had a median BMI of 43, and nearly half also had diabetes.
Heavy menstrual bleeding was the most common pattern across all diagnoses, occurring in 63 percent of cases. Notably, about half of the women ultimately diagnosed with hyperplasia or cancer had previously tried and failed other treatments for their bleeding before the underlying problem was identified. That’s an important detail: bleeding that doesn’t improve with standard treatment is a reason to push for further testing, not to assume it’s just stubborn.
Postmenopausal bleeding deserves special attention. Approximately 9 percent of postmenopausal women who see a doctor for bleeding are eventually diagnosed with endometrial cancer. That means the large majority have a benign explanation, often thinning of the uterine lining (atrophy), polyps, or hormone therapy effects. But because the risk is real and screening is straightforward, any bleeding after menopause should be evaluated.
Bleeding Related to Pregnancy
Unexpected bleeding can sometimes be the first sign of a pregnancy complication, including miscarriage, ectopic pregnancy (where a fertilized egg implants outside the uterus), or problems with the placenta. Even light spotting during a known pregnancy warrants prompt attention. In some cases, bleeding occurs before a person even realizes they’re pregnant, which is why a pregnancy test is typically one of the first steps when evaluating abnormal bleeding in anyone of reproductive age.
Bleeding Disorders
A small but significant number of people with very heavy periods have an underlying bleeding disorder, most commonly von Willebrand disease. These conditions affect the blood’s ability to clot normally and often show up as heavy periods starting from the very first cycle in adolescence. Clues include easy bruising, prolonged bleeding from cuts, and heavy bleeding after dental work or surgery. If your periods have been excessively heavy since they started, a bleeding disorder is worth considering.
How the Cause Is Found
Evaluation usually starts with a detailed history of your bleeding pattern, a physical exam, and blood tests (including a pregnancy test and sometimes thyroid or clotting studies). The next step depends on your age and risk factors. If a structural problem like fibroids or polyps is suspected, especially when previous treatment hasn’t worked, a transvaginal ultrasound is the standard first-line imaging test. If the ultrasound doesn’t show a clear picture of the uterine cavity, a more detailed procedure using saline to expand the uterus (sonohysterography) or a direct camera view (hysteroscopy) may follow.
For people at increased risk of endometrial hyperplasia or cancer, which includes those with obesity, a long history of irregular cycles, or age over 45, an endometrial biopsy is often recommended. This is a quick in-office procedure where a thin tube collects a small sample of the uterine lining for microscopic analysis. In rare situations where these approaches aren’t possible or conclusive, MRI may be used.