A normal menstrual cycle runs between 21 and 37 days from the first day of one period to the first day of the next. If your cycles are consistently shorter than 21 days, that’s considered abnormally frequent, a pattern doctors call polymenorrhea. But even cycles that have recently shortened from your personal norm (say, dropping from 30 days to 22) are worth paying attention to, because something has shifted in the hormonal chain that controls your cycle.
Several things can cause this, ranging from harmless life-stage transitions to treatable medical conditions. Here’s what might be going on.
Hormonal Imbalance and Missed Ovulation
The most common reason for frequent periods is a disruption in ovulation. In a typical cycle, your ovary releases an egg, and the empty follicle it leaves behind (the corpus luteum) produces progesterone. That progesterone stabilizes your uterine lining and keeps it in place until levels drop, triggering a predictable period. When ovulation doesn’t happen, the corpus luteum never forms, so progesterone stays low. Your uterine lining keeps building under the influence of estrogen alone, becomes unstable, and sheds unpredictably, sometimes more often than it should.
This type of bleeding can also be heavier than usual. Without progesterone to counterbalance it, estrogen weakens blood vessels in the uterine lining and reduces their ability to contract, which increases blood loss. Stress, rapid weight changes, excessive exercise, and illness can all temporarily disrupt ovulation and create this pattern.
Perimenopause and Cycle Shortening
If you’re in your late 30s or 40s, shorter cycles may simply be perimenopause. This transition phase, when ovarian function gradually declines, typically lasts two to four years before periods stop entirely. The average age of menopause in Europe and North America is around 51, but the hormonal shifts that precede it can start years earlier. European clinical guidelines recommend considering perimenopause when menstrual irregularity appears in women as young as 40.
During this phase, cycles often get shorter before they get longer and eventually disappear. You might notice periods arriving every 18 to 24 days for a stretch, then skipping a month entirely. Hot flashes, night sweats, and sleep disruptions sometimes accompany these cycle changes, but not always. Some women notice only the shift in their periods at first. If you’re under 40 and experiencing this pattern along with other symptoms like hot flashes or difficulty conceiving, premature ovarian insufficiency is a possibility your doctor can test for.
Thyroid Problems
Your thyroid gland plays a quieter but real role in menstrual regularity. Both an overactive and underactive thyroid are linked to more menstrual disturbances compared to women with normal thyroid function. Hyperthyroid women tend to have lighter but potentially more frequent bleeding, in part because elevated thyroid hormones raise estrogen levels throughout the cycle. Hypothyroid women more commonly experience heavy or prolonged periods and longer gaps between cycles, though the overlap between these patterns is significant. If your cycle changes come with fatigue, unexplained weight shifts, or feeling unusually cold or warm, a thyroid check is a reasonable step.
Uterine Polyps and Fibroids
Structural growths inside the uterus can cause bleeding that mimics frequent periods. Uterine polyps are soft tissue growths that attach to the uterine wall, ranging from the size of a sesame seed to a golf ball. They cause bleeding between periods, unpredictable periods of varying length and heaviness, and sometimes very heavy flow. Because this bleeding can happen outside your normal cycle window, it may feel like you’re getting your period every two weeks when what’s actually happening is intermenstrual bleeding on top of your regular cycle.
Fibroids, which are noncancerous muscle growths in the uterine wall, cause a similar pattern. Both conditions are common and treatable, and an ultrasound is usually the first step in identifying them.
PCOS Isn’t Always Missed Periods
Polycystic ovary syndrome is best known for causing infrequent or absent periods, with many women going 35 or more days between cycles. But menstrual irregularity in PCOS can go in either direction. Some women with PCOS experience two to three periods per month or even continuous bleeding. This happens through the same mechanism described above: without regular ovulation, the lining builds unevenly and sheds at unpredictable intervals. If your frequent periods come with acne, excess hair growth, or difficulty losing weight, PCOS is worth discussing with a provider.
Hormonal Contraception Side Effects
Starting or switching hormonal birth control is one of the most straightforward explanations for frequent bleeding. Roughly 40% of women who use progestin-only pills experience irregular vaginal bleeding during the first three to six months. This can look like spotting between periods, shorter cycles, or bleeding that comes and goes without a clear pattern. Hormonal IUDs, implants, and injections can cause the same adjustment period. In most cases the bleeding settles down after the first few months, but if it persists or is heavy enough to be disruptive, your prescriber can discuss alternatives.
What Doctors Look For
When you bring up frequent periods, the evaluation usually starts with a blood draw. The key tests check your hemoglobin level (to see if you’re becoming anemic from the extra blood loss), your iron stores, and your thyroid function. If there’s reason to suspect a clotting disorder, that may be tested as well.
A transvaginal ultrasound is typically the first imaging step to look for structural problems like polyps, fibroids, or ovarian cysts. If the ultrasound is inconclusive, a saline infusion sonography (where sterile fluid fills the uterus to give a clearer picture) or a hysteroscopy (a thin camera inserted through the cervix) can provide more detail. For women over 45, an endometrial biopsy is often recommended to rule out abnormal cell changes in the uterine lining.
How Frequent Periods Are Treated
Treatment depends entirely on the underlying cause. For hormonal imbalances, the goal is usually restoring the progesterone your body is missing. Oral progesterone supplements can stabilize the uterine lining and create a more predictable cycle. Combined oral contraceptives do the same thing by providing both estrogen and progesterone in controlled doses. A hormonal IUD thins the uterine lining directly and reduces both the frequency and heaviness of bleeding for many women.
If bleeding is heavy, anti-inflammatory medications like ibuprofen or naproxen can reduce blood loss and ease cramping. Tranexamic acid, a prescription medication taken only during bleeding days, also reduces flow. For women who are already anemic or trending that way, iron supplements are an important part of recovery. Fatigue, shortness of breath, and feeling drained during your period can all be signs your iron is low.
When polyps or fibroids are the cause, removal of the growth often resolves the bleeding. For thyroid conditions, treating the thyroid imbalance itself typically brings cycles back toward normal.
Signs Your Bleeding Needs Prompt Attention
Frequent periods alone are worth a medical visit, but certain patterns signal more urgency. Needing to change your pad or tampon more often than every two hours, soaking through one or more pads per hour for several consecutive hours, passing blood clots the size of a quarter or larger, or bleeding that lasts longer than seven days per episode all qualify as heavy menstrual bleeding. Constant lower abdominal pain during periods, persistent fatigue, and shortness of breath are also red flags, particularly because they can indicate anemia that’s been building quietly over several cycles.