A period that’s suddenly heavier than usual is almost always a sign that something has shifted in your hormones, your uterine lining, or both. The cause can range from a one-off anovulatory cycle (where you didn’t ovulate that month) to structural changes like fibroids or polyps. Clinically, bleeding is considered heavy when you lose more than 80 mL per cycle or bleed for longer than seven days, but you don’t need to measure that precisely to know something is off.
How to Tell If Your Bleeding Is Actually Heavy
Most people can’t estimate their blood loss in milliliters, and you don’t need to. The CDC offers more practical benchmarks: if you’re soaking through a pad or tampon in less than two hours, passing large clots, doubling up on pads, or waking up at night to change protection, your flow qualifies as heavy. A single heavy day mid-period is common and not necessarily a concern. The pattern that matters is when heavy flow lasts for several hours in a row or when your period stretches beyond seven days.
It also helps to compare against your own baseline. If your periods have been predictable for years and one cycle is noticeably different, that shift itself is worth paying attention to, even if you’re not hitting every clinical threshold.
Hormonal Imbalance Is the Most Common Cause
Your menstrual cycle relies on a back-and-forth between estrogen and progesterone. In the first half of the cycle, estrogen thickens the uterine lining to prepare for a potential pregnancy. After ovulation, progesterone stabilizes that lining. When progesterone drops at the end of the cycle, the lining sheds and you get your period.
The problem starts when you don’t ovulate. Without ovulation, your body never produces that stabilizing wave of progesterone, so estrogen keeps building the lining unopposed. By the time bleeding finally starts, there’s significantly more tissue to shed, which means a heavier, longer, and often more painful period. This is called endometrial hyperplasia when the lining becomes abnormally thick.
Skipped ovulation happens more often than most people realize. Stress, sudden weight changes, illness, travel, and disrupted sleep can all delay or prevent ovulation in a given cycle. If it happens once, you get one unusually heavy period and then things return to normal. If it becomes a pattern, the heavy bleeding tends to recur.
PCOS and Chronic Anovulation
Polycystic ovary syndrome is one of the most common reasons for repeatedly heavy or irregular periods in people of reproductive age. In PCOS, elevated androgen levels prevent the ovaries from releasing eggs regularly. The result is the same mechanism described above: without ovulation, progesterone stays low, estrogen continues stimulating the uterine lining, and when a period finally arrives (sometimes after weeks or months of delay), it tends to be heavy.
PCOS also raises the long-term risk of endometrial hyperplasia because the lining spends so much time growing without being shed. Other signs that point toward PCOS include acne, excess facial or body hair, and difficulty losing weight, though not everyone with the condition has visible symptoms.
Fibroids, Polyps, and Adenomyosis
Structural growths in or on the uterus are another major category. These are almost always benign, but they physically change how the uterus bleeds.
- Fibroids are smooth muscle tumors that grow in the uterine wall. They’re extremely common, especially after age 30, and their effect on bleeding depends largely on location. Fibroids that press into the uterine cavity tend to cause the heaviest flow because they distort the lining and increase its surface area.
- Polyps are small, soft overgrowths of the uterine lining itself. They’re estimated to be present in 10 to 24 percent of women who undergo uterine biopsy or surgery. Polyps are rare before age 20 and peak in frequency during the 40s. They can cause heavy bleeding, spotting between periods, or both.
- Adenomyosis occurs when tissue from the uterine lining embeds itself into the muscular wall of the uterus. This makes the uterus enlarged and boggy, which leads to heavier, more painful periods. It most commonly affects women between 40 and 50 who have previously given birth. About half of people with adenomyosis also have fibroids.
All three conditions can exist for years before symptoms become noticeable. A period that gradually gets heavier over several cycles, rather than changing abruptly, is a typical pattern with structural causes.
Your IUD Could Be a Factor
If you have a copper (non-hormonal) IUD, heavier periods are a well-documented side effect. Research shows that copper IUDs increase menstrual blood loss by roughly 55 percent compared to pre-insertion levels. For most people, this increase levels off after the first few months, but it doesn’t fully go away as long as the device is in place. Hormonal IUDs, by contrast, typically make periods lighter over time, so a sudden increase in bleeding with a hormonal IUD is worth investigating for other causes.
Bleeding Disorders Are Underdiagnosed
Heavy periods that have been present since your very first cycle, or that run in your family, may point to an inherited bleeding disorder. Von Willebrand disease is the most common one. Among women with chronic heavy menstrual bleeding, somewhere between 5 and 24 percent turn out to have von Willebrand disease, yet many go undiagnosed for years because heavy periods get normalized as “just how it is.”
Clues that a bleeding disorder might be involved include easy bruising, prolonged bleeding after dental work or minor cuts, and heavy bleeding after childbirth. If you’ve been told your periods are heavy but every ultrasound and hormone test comes back normal, a bleeding disorder workup is a reasonable next step.
Other Triggers Worth Knowing
Several less obvious factors can make a single period heavier than expected. Starting or stopping hormonal birth control disrupts the hormonal rhythm your body has adapted to, and breakthrough heavy bleeding is common during the transition. Perimenopause, which can begin in your early 40s (or occasionally late 30s), brings increasingly erratic ovulation, so heavy cycles interspersed with lighter ones are a hallmark of that transition. Thyroid disorders, particularly an underactive thyroid, can also slow ovulation and contribute to heavier flow. Even significant emotional stress or rapid weight loss can suppress ovulation for a cycle or two, producing the same lining-buildup effect.
When Heavy Bleeding Becomes a Health Problem
The most common consequence of ongoing heavy periods is iron deficiency. Every period depletes your iron stores, and heavy bleeding accelerates the loss faster than most diets can replace it. Iron deficiency shows up as fatigue, brain fog, shortness of breath during light activity, brittle nails, and feeling cold all the time. You can be iron deficient well before you’re technically anemic. A ferritin level below 30 ng/mL indicates depleted iron stores, and hemoglobin below 12 g/dL in women signals anemia.
If you’ve been dealing with heavy periods and recognize those symptoms, it’s worth getting your ferritin checked specifically. A standard blood count can miss early iron depletion because hemoglobin doesn’t drop until your stores are significantly drained.
What Happens During Evaluation
Figuring out the cause usually starts with a detailed history of your cycle patterns, then blood work to check hormone levels, thyroid function, and iron stores. A pelvic ultrasound is the standard imaging tool, used to look for fibroids, polyps, ovarian cysts, and unusual thickening of the uterine lining. If the ultrasound raises questions, a more detailed look inside the uterus with a saline-infusion sonogram or a small camera (hysteroscopy) can clarify what’s there. In some cases, a small tissue sample from the lining is taken to rule out hyperplasia or precancerous changes, particularly in women over 35 or those with prolonged irregular bleeding.
Treatment depends entirely on the underlying cause. Hormonal imbalances and anovulatory cycles often respond well to hormonal management. Fibroids and polyps can sometimes be removed in a straightforward outpatient procedure. Iron deficiency is treated alongside the bleeding itself, since resolving one without the other leaves you stuck in the same cycle.