Abnormal uterine bleeding (AUB) has many possible causes, ranging from hormonal imbalances and uterine growths to blood clotting disorders and certain medications. It affects roughly 3% to 30% of women of reproductive age and accounts for about 30% of all gynecological visits, a number that climbs to 70% among women approaching or past menopause. Despite how common it is, nearly half of women with abnormal bleeding never seek medical help.
Understanding the cause matters because treatment depends entirely on it. A hormonal issue calls for a very different approach than a structural growth or a bleeding disorder.
What Counts as Abnormal Bleeding
Normal menstrual blood loss is generally under 60 mL per cycle. Bleeding is considered moderately heavy between 60 and 100 mL, and excessive above 100 mL. The older clinical cutoff for “too heavy” was 80 mL, but in practice, what matters more than measuring milliliters is the pattern: soaking through a pad or tampon every hour, passing large clots, bleeding that lasts longer than seven days, or cycles that arrive unpredictably.
Abnormal bleeding also includes spotting between periods, bleeding after sex, and any bleeding after menopause. Postmenopausal bleeding always needs evaluation, even if it’s light.
Structural Causes
The international classification system used by gynecologists groups causes into two broad categories. The first is structural, meaning something physically changed in the uterus that can usually be seen on imaging.
Polyps
Uterine polyps are small, soft growths on the inner lining of the uterus. They’re usually noncancerous but can cause bleeding between periods or heavier-than-normal flow. They become more common with age and are frequently found in women on hormone replacement therapy or tamoxifen.
Adenomyosis
In adenomyosis, tissue that normally lines the uterus grows into the muscular wall. This makes the uterus enlarged and tender, and typically causes heavy, prolonged, painful periods. It’s most common in women in their 30s and 40s who have had children, though it can occur earlier.
Fibroids
Fibroids (also called leiomyomas) are noncancerous muscle growths in or on the uterus. They’re extremely common. Not all fibroids cause bleeding, but those that grow into or distort the uterine cavity tend to produce heavy periods, prolonged bleeding, and sometimes pelvic pressure. Their size ranges from a few millimeters to grapefruit-sized or larger.
Endometrial Hyperplasia and Cancer
An overgrowth of the uterine lining, called endometrial hyperplasia, can cause heavy or irregular bleeding and sometimes progresses to cancer. Among women 45 and younger evaluated for abnormal bleeding, about 1.6% are found to have endometrial cancer. The risk factors are specific: higher body weight (the median BMI in one study of women with endometrial cancer was 43), diabetes, high blood pressure, and a history of failed medical treatment for abnormal bleeding. Heavy menstrual bleeding was the most common pattern, followed by infrequent periods.
These numbers are reassuring in that cancer is an uncommon cause of abnormal bleeding, especially in younger women. But they also explain why doctors take certain risk factors seriously and may recommend a biopsy sooner rather than later.
Non-Structural Causes
The second category includes causes you can’t see on an ultrasound or during surgery. These involve hormones, blood clotting, the uterine lining itself, or medications.
Ovulatory Disorders
This is one of the most common causes of abnormal bleeding, particularly in teenagers and women approaching menopause. When ovulation doesn’t happen regularly, progesterone levels stay low. Without progesterone to stabilize the uterine lining, estrogen builds it up unevenly, leading to irregular, sometimes very heavy shedding.
Polycystic ovary syndrome (PCOS) is a major driver of ovulatory dysfunction. In PCOS, the ovaries produce unusually high levels of androgens (male-type hormones), which prevent regular egg release. Insulin resistance plays a central role: elevated insulin triggers the ovaries to produce more androgens, which suppress ovulation and disrupt the menstrual cycle. Obesity worsens insulin resistance, creating a cycle that amplifies hormonal imbalance. Other contributors to irregular ovulation include thyroid disorders, significant weight changes, chronic stress, and excessive exercise.
Blood Clotting Disorders
Some women bleed heavily because their blood doesn’t clot properly. Von Willebrand disease, the most common inherited bleeding disorder, is found in 5% to 24% of women with chronic heavy periods. It’s more prevalent among white women (about 16%) than Black women (about 1%). Many of these women go undiagnosed for years because heavy periods get attributed to other causes. If your heavy bleeding started with your very first period, runs in your family, or you bruise easily and bleed a long time from cuts, a clotting disorder is worth investigating.
Endometrial Disorders
Sometimes the uterine lining itself doesn’t function normally, even without polyps, hyperplasia, or hormonal problems. The lining may not repair efficiently after shedding, or local inflammation may cause bleeding. This category is less well understood and is often a diagnosis made after other causes have been ruled out.
Medications
Several common medications can trigger abnormal bleeding. Blood thinners and aspirin reduce clotting and can make periods heavier or cause spotting. Hormone replacement therapy and certain types of birth control, especially in the first few months, frequently cause irregular bleeding. Tamoxifen, used for breast cancer, stimulates the uterine lining and can cause bleeding. Intrauterine devices (IUDs) may cause irregular spotting, particularly hormonal IUDs in the first three to six months. If you’ve recently started or changed a medication and notice new bleeding patterns, that connection is worth flagging.
Why Bleeding After Menopause Is Different
Any bleeding after menopause needs its own evaluation because the possible causes shift. The uterine lining should be thin and inactive without estrogen stimulation, so bleeding signals that something is triggering growth or irritation. Causes range from benign (vaginal dryness, polyps, hormone therapy) to serious (endometrial hyperplasia or cancer).
An ultrasound measuring the uterine lining is typically the first step. A lining of 4 mm or less has a greater than 99% negative predictive value for cancer, meaning it’s extremely unlikely to be missed. If the lining measures thicker than 4 mm, or if it can’t be clearly measured, further evaluation with a biopsy or more detailed imaging is needed. One important caveat: rare types of endometrial cancer can develop even with a thin lining (under 3 mm), so any postmenopausal bleeding that persists or returns needs a tissue sample regardless of what the ultrasound shows.
How the Cause Is Found
The diagnostic path depends on your age, symptoms, and risk factors. A medical history and physical exam come first. Your doctor will ask about the pattern of bleeding (heavy, prolonged, irregular, between periods), how long it’s been happening, your medications, and your family history of bleeding disorders or cancer.
If there’s concern about the uterine lining, particularly in women with risk factors like obesity, diabetes, or long stretches without periods, an endometrial biopsy is usually recommended. This is a brief office procedure where a thin tube collects a small tissue sample from the lining.
When a structural problem is suspected, especially if previous treatment hasn’t worked, imaging starts with a transvaginal ultrasound. If the ultrasound doesn’t show a clearly normal cavity, the next step is usually a saline-infusion sonogram (where fluid is placed in the uterus during the ultrasound to get a clearer picture) or hysteroscopy (a thin camera inserted through the cervix). In rare cases where these aren’t feasible, MRI may be used.
Blood tests to check for anemia, thyroid problems, clotting disorders, and pregnancy round out the workup. For younger women and teens with heavy periods from the start, screening for von Willebrand disease and other clotting conditions is particularly important since these are frequently missed.
The Impact Beyond Bleeding
Abnormal bleeding carries consequences that go well beyond inconvenience. Chronic heavy periods cause iron-deficiency anemia, which leads to fatigue, brain fog, and shortness of breath. The unpredictability of bleeding affects work attendance, school participation, and sexual health. Women entering pregnancy already anemic from years of heavy periods face higher risks of complications. The financial burden of pads, tampons, doctor visits, and missed work adds up quietly over months and years.
Because the causes are so varied, there’s no single fix. But identifying the specific cause is the step that makes effective treatment possible, whether that’s hormonal management for ovulatory dysfunction, removal of a polyp or fibroid, treatment for a clotting disorder, or a medication adjustment.