An irregular period is any menstrual cycle that falls outside the typical range of 21 to 35 days, or one that varies significantly in length from month to month. Roughly 14% to 25% of women of childbearing age experience some form of menstrual irregularity, making it one of the most common reproductive health concerns. While a cycle that’s a few days off once in a while is rarely cause for alarm, persistent irregularity often signals something worth investigating.
What Counts as a Normal Cycle
A “normal” menstrual cycle repeats every 21 to 35 days, with bleeding that lasts between 2 and 7 days. The count starts on the first day of one period and ends on the first day of the next. Most people settle into a pattern where their cycle length stays relatively consistent, give or take a couple of days.
A period crosses into irregular territory when cycles consistently fall shorter than 21 days or longer than 35 days, when you skip periods entirely without being pregnant, or when your cycle length swings unpredictably from one month to the next. Bleeding that lasts longer than 7 days or requires you to change a pad or tampon every one to two hours also qualifies as abnormal, even if the timing itself seems regular. Blood loss exceeding about 80 mL per cycle (roughly soaking through a pad every hour or two for several hours) is considered heavy.
Why Irregular Periods Happen
The menstrual cycle depends on a precise chain of hormonal signals between the brain and the ovaries. When any link in that chain is disrupted, ovulation can be delayed, skipped, or thrown off schedule, and the period follows suit. The causes fall into two broad categories: structural issues (physical changes in the uterus) and non-structural issues (hormonal or systemic problems).
Hormonal and Systemic Causes
Polycystic ovary syndrome (PCOS) is one of the most common reasons for irregular or missing periods. In PCOS, elevated levels of androgens (sometimes called “male hormones,” though everyone produces them) interfere with the normal development and release of eggs. Without regular ovulation, periods become unpredictable or stop altogether.
Thyroid disorders are another frequent culprit. An underactive thyroid disrupts the hormonal signals that trigger ovulation in several ways: it raises prolactin levels (the hormone associated with breastfeeding), which suppresses the brain’s signal to start the ovulation process. It also interferes with how the ovaries respond to growth signals, sometimes leading to cyst formation. Women with PCOS have roughly 3.6 times higher odds of also having an underactive thyroid compared to women without PCOS, which can compound cycle problems.
Other non-structural causes include significant stress, rapid weight loss or gain, eating disorders, excessive exercise, and certain medications, particularly hormonal contraceptives during the adjustment period.
Structural Causes
Physical changes inside the uterus can also alter bleeding patterns. Polyps (small tissue growths on the uterine lining), fibroids (noncancerous muscle tumors in the uterine wall), and a condition called adenomyosis (where uterine lining tissue grows into the muscular wall) can all cause heavier, longer, or more frequent bleeding. In rare cases, precancerous or cancerous changes to the uterine lining are responsible.
How Your Age Affects Regularity
Irregular periods are especially common at both ends of your reproductive years, and in both cases the irregularity is a normal part of the transition.
During the first year or so after a first period, the hormonal communication system between the brain and ovaries is still calibrating. Cycles during early adolescence can be long, short, or unpredictable while the body learns to produce hormones at consistent levels. Most teens settle into a more regular pattern within one to two years.
On the other end, perimenopause (the transition leading up to menopause, typically in the mid-to-late 40s) brings its own wave of irregularity. As the supply of eggs dwindles, the remaining ones produce less estrogen. The brain compensates by ramping up its ovulation-triggering signals, which can shorten cycles to as little as 21 days. Skipped periods become increasingly common as ovulation becomes sporadic. This transition can last several years before periods stop entirely.
Signs That Need Medical Attention
Some irregularities are worth bringing to a doctor sooner rather than later. You should schedule an appointment if:
- Your periods were previously regular and have become irregular
- Your cycle is consistently shorter than 24 days or longer than 38 days
- You go three or more months without a period and are not pregnant or breastfeeding
- Your period lasts longer than 8 days
- You soak through a pad or tampon every one to two hours for several hours
- You pass blood clots larger than a quarter
- You experience bleeding after sex on more than one occasion
- You feel dizzy, lightheaded, weak, or have chest pain during or after your period
- You have any bleeding after menopause
For teens, it’s worth seeing a doctor if periods haven’t started by age 15, or if breast development hasn’t begun by age 13.
How Irregular Periods Are Diagnosed
Tracking your cycles for a few months before your appointment gives your doctor useful data to work with. Note the start date, duration, flow level, and any symptoms for each cycle.
A standard workup typically starts with blood tests to check hormone levels. These commonly include thyroid hormones, prolactin, and reproductive hormones like FSH (which triggers egg development), LH (which triggers ovulation), estrogen, progesterone, and testosterone. Your doctor interprets these results together, since the ratios between hormones matter as much as the individual numbers. Your age, symptoms, and medical history all factor into the interpretation.
Depending on what the bloodwork suggests, imaging like an ultrasound may be used to look for structural causes such as polyps, fibroids, or ovarian cysts.
Treatment Options
Treatment depends entirely on the underlying cause and whether pregnancy is something you’re currently planning.
For cycles that are irregular because of missed or infrequent ovulation, hormonal options are the most common approach. Birth control pills regulate the cycle by providing consistent hormone levels, while cyclic progesterone therapy can trigger a period on a predictable schedule without suppressing ovulation long-term. If an underlying condition like a thyroid disorder or an eating disorder is driving the irregularity, treating that condition directly often restores regular cycles on its own.
For heavy or prolonged bleeding that occurs on a roughly regular schedule, a hormone-releasing IUD can thin the uterine lining and significantly reduce flow. Anti-inflammatory medications can also help with both heavy bleeding and cramping.
When structural issues like polyps or fibroids are responsible and don’t respond to medication, surgical options range from minimally invasive removal of the growth to procedures that reduce the uterine lining. These are typically considered only after other approaches haven’t worked.
Lifestyle factors play a supporting role regardless of the cause. Maintaining a stable weight, managing stress, and avoiding extreme exercise patterns all help keep the hormonal chain of command functioning smoothly. For people with PCOS in particular, even modest changes in weight can measurably improve cycle regularity.