Irregular bleeding is any menstrual bleeding that falls outside normal patterns in timing, duration, or volume. A normal cycle ranges from 21 to 35 days, with bleeding lasting four to seven days. If your periods come closer together than every 21 days, more than 35 days apart, last longer than a week, or vary by more than nine days from one cycle to the next, that counts as irregular bleeding. Spotting between periods, unusually heavy flow, and missed periods all fall under this umbrella too.
What Counts as Irregular
The clinical definition covers a wider range of patterns than most people expect. Missing three or more periods in a row qualifies, but so does something subtler: if one cycle is 28 days, the next is 37, and the one after that is 29, that nine-day swing is considered irregular even though each cycle individually falls within the normal range. Flow that’s noticeably heavier or lighter than your usual pattern also counts, even if it arrives on schedule.
Heavy bleeding has a specific threshold. Soaking through a tampon or pad every hour is a sign your flow has crossed into abnormally heavy territory. If that pace continues for more than two hours and you also feel lightheaded, dizzy, short of breath, or have chest pain, that’s a situation requiring emergency care.
Why It Happens: Hormonal Causes
Most irregular bleeding traces back to hormones. Your uterine lining builds up and sheds in response to estrogen and progesterone, and when those hormones fall out of balance, the lining becomes unstable. Too much estrogen without enough progesterone causes the lining to overgrow, creating a fragile structure that sheds in unpredictable patches rather than all at once. This is the mechanism behind a lot of the spotting and prolonged bleeding people experience.
Polycystic ovary syndrome (PCOS) is one of the most common hormonal culprits. It disrupts ovulation, meaning progesterone never rises to stabilize the lining, and cycles become long, absent, or unpredictable. Thyroid problems are another major player that often gets overlooked. In one study of women with menstrual irregularities, 55% of those with elevated thyroid-stimulating hormone levels experienced infrequent periods, while 65% of those with low thyroid hormone levels had abnormally heavy bleeding. Autoimmune thyroid conditions were also strongly linked to missed periods entirely.
Structural Causes Inside the Uterus
Sometimes the problem isn’t hormonal but physical. Uterine polyps, which are small growths on the inner lining, are estrogen-sensitive and can cause bleeding between periods. Fibroids, which are noncancerous muscle growths in or on the uterine wall, can increase menstrual flow and make periods last longer. A condition called adenomyosis, where lining tissue grows into the muscular wall of the uterus, produces similar symptoms: heavier, more painful periods.
These structural causes are important to identify because they often don’t respond to the same treatments as hormonal irregularities, and some require procedural intervention.
Medications That Can Trigger Bleeding
Hormonal contraceptives are the single most common medication-related cause of irregular bleeding. This includes birth control pills, hormonal IUDs, and estrogen replacement therapy. When you take hormonal contraceptives, the balance between estrogen and progestin directly affects lining stability. Too much progestin over time thins the lining so much that small areas detach unpredictably, causing spotting. Too much estrogen makes the lining proliferate beyond what the underlying tissue can support, and fragile new blood vessels rupture.
Other medications cause irregular bleeding through entirely different pathways. Blood thinners and antiplatelet drugs reduce your blood’s ability to clot, which can make periods heavier or cause spotting. Certain antidepressants, including SSRIs, tricyclic antidepressants, and antipsychotic medications, can disrupt menstrual patterns. Even herbal supplements like ginseng, soy protein, and ginkgo biloba have been linked to breakthrough bleeding in some women.
Irregular Bleeding During Perimenopause
If you’re in your 40s or early 50s, irregular bleeding is almost a given. More than 90% of women experience at least one episode of abnormal bleeding during the transition to menopause, and 78% have three or more episodes. This happens because ovulation becomes inconsistent, estrogen levels swing unpredictably, and the uterine lining responds to those fluctuations with erratic shedding. Cycles may come closer together, then space out, then arrive with unusually heavy flow.
The challenge during perimenopause is distinguishing normal transitional changes from something that needs attention. Bleeding that’s heavy enough to soak a pad or tampon every hour, bleeding that occurs after a gap of 12 months or more (which would be post-menopausal bleeding), or bleeding that persists despite treatment all warrant investigation, because the risk of endometrial abnormalities increases with age.
How Irregular Bleeding Is Diagnosed
Diagnosis starts with blood work to check hormone levels, thyroid function, and sometimes clotting factors. If a structural cause is suspected, or if bleeding persists after initial treatment, ultrasound is the first-line imaging tool. A transvaginal ultrasound can identify polyps, fibroids, and signs of adenomyosis. In some cases, saline is infused into the uterine cavity during ultrasound to get a clearer picture of growths inside the lining.
For women 45 and older, an endometrial biopsy is standard because age raises the risk of endometrial cancer. Younger women may also need a biopsy if they have persistent bleeding, a history of prolonged periods without ovulation (which exposes the lining to estrogen without the protective counterbalance of progesterone), or if initial treatments haven’t worked. The biopsy is typically done in an office setting and involves collecting a small tissue sample from the uterine lining.
Treatment Options
Treatment depends entirely on the cause. For hormonal imbalances and cycles where ovulation isn’t happening regularly, progesterone-based therapy is the most common approach. Cyclical progesterone, taken during the second half of the cycle, triggers a predictable withdrawal bleed and prevents the lining from building up excessively. Continuous progesterone works differently, thinning the lining over time to reduce or stop bleeding altogether. A hormonal IUD delivers progesterone directly to the uterus and is effective for both heavy and irregular bleeding.
If thyroid dysfunction is the underlying cause, treating the thyroid condition itself often resolves the menstrual irregularity without any additional gynecological intervention. The same principle applies to medication-induced bleeding: switching to a different contraceptive formulation, adjusting a dose, or discontinuing a supplement may be all that’s needed.
For structural problems like polyps or fibroids, the approach shifts toward removal. Polyps can often be removed during a hysteroscopy, a procedure where a small camera is inserted through the cervix. Fibroids have a wider range of options depending on their size and location. For women who have completed childbearing and haven’t responded to other treatments, endometrial ablation (which destroys the uterine lining) or hysterectomy may be considered, though these are typically last-resort options.