A period that’s suddenly longer or heavier than your normal pattern usually signals a hormonal shift, but it can also point to structural changes in the uterus, a thyroid problem, or even an undiagnosed bleeding disorder. One unusually heavy cycle after stress, illness, or a skipped ovulation is common and often resolves on its own. When the change persists for two or more cycles, something worth investigating is likely driving it.
Clinically, bleeding counts as “heavy” if it soaks through a pad or tampon every hour for several consecutive hours, lasts longer than seven days, produces clots the size of a quarter or larger, or forces you to double up on pads or change them overnight.
Skipped Ovulation and Hormone Imbalance
The most common reason for an unexpectedly heavy period is a cycle where you didn’t ovulate. After an egg is released, the empty follicle produces progesterone, which stabilizes the uterine lining and keeps it from growing too thick. When ovulation doesn’t happen, progesterone never rises. Estrogen continues building the lining unopposed, sometimes for weeks, until it finally sheds in a heavier, messier bleed than usual.
This can happen to anyone occasionally. A stressful month, jet lag, significant weight change, or a bout of illness can delay or cancel ovulation without meaning anything is chronically wrong. But if it keeps happening, conditions like polycystic ovary syndrome (PCOS) may be involved. People with PCOS often go a month or two with no bleeding at all, then experience a period that’s far heavier than normal, precisely because the lining has been thickening without progesterone to keep it in check.
Fibroids and Polyps
Uterine fibroids are noncancerous growths in the muscle wall of the uterus. They’re extremely common, especially after age 30, and many cause no symptoms at all. But fibroids that grow near the inner lining can distort the uterine cavity, increase the surface area that bleeds each month, and interfere with the uterus’s ability to contract and stop bleeding. The result is periods that drag on longer or produce noticeably more flow.
Uterine polyps work differently. These are overgrowths of the endometrial lining itself, and they’re estrogen-sensitive, meaning they grow in response to estrogen circulating in your body. Polyps cause irregular bleeding, very heavy flow, and sometimes spotting between periods. They’re typically small and benign, but they don’t resolve on their own the way a one-off hormonal blip does.
Adenomyosis
Adenomyosis is a condition where tissue that normally lines the uterus starts growing into the muscular wall. It causes heavy periods, large clots, and cramping that can feel deep and relentless. The uterus itself often becomes enlarged and tender. Unlike endometriosis, which grows outside the uterus and tends to flare mainly during your period, adenomyosis can cause pressure, bloating, and pelvic fullness throughout your entire cycle.
Adenomyosis is most commonly diagnosed in people in their 30s and 40s, particularly those who have had pregnancies or uterine surgery. It’s frequently missed on standard exams because the uterus may just feel “bulky” without an obvious mass. An ultrasound or MRI is usually needed to identify it.
Thyroid Problems
An underactive thyroid disrupts your period through several pathways at once. Thyroid hormones interact directly with the reproductive hormone system. When thyroid levels drop, the pituitary gland overproduces thyroid-stimulating hormone (TSH) to compensate. That overproduction can suppress the hormones that trigger ovulation, leading to the same unopposed estrogen buildup described above, which eventually causes heavy breakthrough bleeding.
Hypothyroidism also impairs blood clotting. It can cause platelet dysfunction and shift the body toward a state where blood doesn’t clot as efficiently, making periods heavier simply because the normal clotting mechanisms that help stop menstrual bleeding aren’t working at full capacity. Even subclinical hypothyroidism, where thyroid levels are only mildly off, has been linked to heavier flow. If your heavier periods coincide with fatigue, weight gain, cold sensitivity, or brain fog, a thyroid panel is a straightforward blood test that can rule this in or out quickly.
Bleeding Disorders
Some people have always had heavy periods and assumed it was normal. In those cases, an inherited bleeding disorder may be the underlying cause. Von Willebrand disease, a condition where the blood lacks enough of a specific clotting protein, is found in 5% to 24% of people with chronic heavy menstrual bleeding. It’s significantly underdiagnosed because heavy periods are often dismissed as “just how things are” rather than investigated.
Clues that a bleeding disorder might be involved include heavy periods starting from your very first cycle, easy bruising, prolonged bleeding after dental work or minor cuts, and a family history of bleeding problems. If your periods have always been heavy rather than recently changing, this is worth bringing up specifically.
Perimenopause
If you’re in your late 30s or 40s, the hormonal transition leading to menopause is one of the likeliest explanations for changes in your cycle. During perimenopause, ovulation becomes less reliable. Some cycles are normal, others skip ovulation entirely, and hormone levels fluctuate unpredictably. This means your periods may come closer together or further apart, and the flow can swing from lighter than usual to significantly heavier.
Perimenopause can last anywhere from a few years to a decade before periods stop entirely. Heavier bleeding during this window is expected to some degree, but it still warrants evaluation if it’s disruptive or meets the clinical thresholds for heavy bleeding. Fibroids and polyps also become more common during this life stage, so a new pattern of heavy bleeding in your 40s can involve overlapping causes.
Other Triggers Worth Knowing
A copper IUD is a well-known cause of heavier, longer periods, particularly in the first three to six months after insertion. If your bleeding change lines up with getting one placed, that’s likely the connection.
Certain medications can also affect flow. Blood thinners, including daily aspirin, reduce your body’s ability to clot and can make periods noticeably heavier. Starting or stopping hormonal birth control often causes temporary changes in cycle length and volume while your body adjusts.
Pregnancy-related causes shouldn’t be overlooked either. An early miscarriage or an ectopic pregnancy can present as a late, unusually heavy period. If your period arrived significantly later than expected and the bleeding is intense or accompanied by sharp pain, a pregnancy test is a reasonable first step.
What Evaluation Looks Like
When you bring up heavier or longer periods, the typical workup starts with blood tests to check for anemia, thyroid function, and sometimes clotting factors. If there’s concern about a structural problem like fibroids or polyps, a transvaginal ultrasound is the standard first imaging step. It’s noninvasive and can identify most growths. If the ultrasound isn’t conclusive, a procedure called sonohysterography (where saline is used to expand the uterine cavity during an ultrasound) or a hysteroscopy (a thin camera inserted through the cervix) gives a clearer picture.
For people at increased risk of endometrial problems, particularly those over 45 or with a long history of irregular cycles, an endometrial biopsy may be recommended. This involves taking a small tissue sample from the uterine lining to check for abnormal cell growth. It’s done in the office and takes a few minutes, though it can cause cramping similar to a bad period.
Tracking your cycle details before your appointment makes the evaluation faster and more productive. Note how many days you bleed, how often you change a pad or tampon, whether you’re passing clots, and any other symptoms like pelvic pain, fatigue, or bleeding between periods. That information helps distinguish between a hormonal issue, a structural one, and a systemic problem like a thyroid or clotting disorder.