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. Your period might also be considered irregular if it lasts longer than seven days, if you skip cycles entirely, or if the flow is dramatically heavier or lighter than usual. Irregular periods are common, and in many cases the cause is identifiable and treatable.
What Counts as a Regular Cycle
A “regular” cycle doesn’t mean it arrives every 28 days like clockwork. Normal menstrual bleeding happens every 21 to 35 days and lasts two to seven days. Some variation from month to month is expected. Your cycle might be 29 days one month and 32 the next, and that’s perfectly normal.
A cycle becomes irregular when it consistently falls shorter than 21 days or longer than 35, when consecutive cycles differ by more than seven days in length, or when you have fewer than eight cycles in a year. Periods that stop entirely for 90 days or more (outside of pregnancy) also qualify. The key distinction is pattern: an occasional off cycle is normal, but a persistent shift signals something worth paying attention to.
Why Periods Are Irregular in Teens
If you’re a teenager or the parent of one, irregular periods in the first few years after a first period are expected. The hormonal system that controls ovulation takes time to mature. By the third year after a first period, only 60 to 80 percent of cycles fall into the typical 21-to-34-day adult range. Before that point, longer gaps between periods, skipped months, and unpredictable timing are all part of the body’s adjustment process.
That said, cycles longer than 45 days in someone one to three years past their first period, or any single cycle longer than 90 days after the first year, fall outside even the expected adolescent range and are worth discussing with a doctor.
Common Causes of Irregular Periods
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common reasons for ongoing irregular periods in people of reproductive age. Diagnosis requires two of three features: irregular cycles, elevated levels of androgens (sometimes called “male hormones,” though everyone produces them), and a characteristic appearance of the ovaries on ultrasound. In practice, many people with PCOS experience long gaps between periods, sometimes going months without one, because ovulation doesn’t happen on a predictable schedule. PCOS also frequently comes with acne, excess hair growth on the face or body, and difficulty maintaining a stable weight.
Thyroid Problems
Your thyroid gland plays a surprisingly large role in menstrual regularity. When thyroid hormone levels are too low (hypothyroidism), the body compensates in ways that raise prolactin, a hormone normally associated with breastfeeding. Elevated prolactin suppresses the brain signals that trigger ovulation, which can delay or stop periods altogether. An overactive thyroid (hyperthyroidism) disrupts the cycle through a different but related pathway, increasing the likelihood of skipped ovulation and unpredictable bleeding. A simple blood test measuring thyroid-stimulating hormone (TSH) can identify the problem.
Stress and Low Body Weight
Your brain constantly monitors your energy balance and stress levels, and when either one signals “not a good time to reproduce,” it can shut down ovulation. High psychological stress raises cortisol and related hormones that directly suppress the brain’s reproductive signaling center. The same thing happens when energy intake is too low relative to energy expenditure, whether from restrictive eating, intense athletic training, or a combination of both. The result is lighter, less frequent, or completely absent periods. This pattern, sometimes called hypothalamic amenorrhea, is reversible once the underlying stressor or energy deficit is addressed.
Perimenopause
Cycle changes are one of the earliest signs of the transition toward menopause. The early stage of perimenopause, defined by a persistent difference of seven or more days between consecutive cycle lengths, begins on average six to eight years before the final menstrual period. That means someone whose last period will happen at 51 might start noticing subtle changes in cycle timing as early as their mid-40s, or even their late 30s.
As perimenopause progresses, cycles become increasingly unpredictable. The late stage is marked by gaps of 60 days or more between periods and begins roughly two years before the final period. Rising levels of follicle-stimulating hormone (FSH) drive these changes as the ovaries gradually produce less estrogen and ovulate less reliably. Heavier bleeding, lighter bleeding, and spotting between periods can all occur during this transition.
Other Causes
Structural issues in the uterus, like polyps (small tissue growths on the uterine lining) or fibroids (noncancerous muscle tumors), can cause heavy or prolonged bleeding that makes cycles feel irregular. Certain medications, including some types of birth control, antidepressants, and blood thinners, can alter bleeding patterns. Significant weight gain or loss, travel, illness, and changes in sleep patterns can all temporarily throw off your cycle as well.
How Irregular Periods Are Evaluated
A doctor’s first step is almost always a pregnancy test, regardless of how unlikely pregnancy seems. From there, the investigation typically involves blood work targeting a handful of key hormones. A thyroid function test checks whether TSH levels are normal. An FSH test evaluates whether the ovaries are functioning as expected, which helps distinguish early menopause from other causes. A prolactin test looks for abnormally high levels that could point to a pituitary issue or thyroid dysfunction. If there are signs like excess facial hair or a deepening voice, androgen levels are checked to screen for PCOS.
In some cases, a doctor may order a hormone challenge test, which involves taking a hormonal medication for seven to ten days to see if it triggers bleeding. The result helps clarify whether the issue is a lack of estrogen production or a problem further upstream in the brain’s signaling chain. Ultrasound imaging may also be used to look for structural issues like polyps, fibroids, or ovarian cysts.
How Irregular Periods Are Managed
Treatment depends entirely on the cause. For PCOS, hormonal birth control is frequently used to establish a predictable cycle, protect the uterine lining from thickening unchecked, and reduce androgen-related symptoms like acne and excess hair growth. Lifestyle changes, particularly maintaining a stable weight and regular physical activity, also improve cycle regularity in many people with PCOS.
For thyroid-related irregularity, treating the underlying thyroid condition usually restores normal cycles. For stress-related or weight-related causes, the path forward involves addressing the root problem: reducing psychological stress, increasing caloric intake, or scaling back exercise intensity.
Hormonal contraceptives are the most widely used tool for cycle regulation across many causes. Combined oral contraceptive pills taken on an extended or continuous schedule (rather than the traditional three-weeks-on, one-week-off pattern) are commonly prescribed to reduce the frequency of bleeding or eliminate it altogether. A hormonal IUD that releases a small amount of progestin locally into the uterus is another option that reduces the intensity, frequency, and duration of menstrual bleeding, and can induce the absence of periods entirely in some users.
For structural causes like polyps or fibroids, the approach ranges from monitoring to minor procedures to remove the growth, depending on the size, location, and severity of symptoms.
Signs That Need Prompt Attention
Most irregular periods aren’t emergencies, but certain patterns warrant a call to your doctor sooner rather than later. If you’re soaking through a pad or tampon every hour for more than two hours in a row, especially if you also feel dizzy, lightheaded, or short of breath, that’s a reason to seek emergency care. Bleeding that persists for more than seven days, spotting between periods or after sex, and any bleeding after menopause all deserve evaluation.
Irregular bleeding that has been happening for six months or more is considered a chronic condition, and tracking your cycles during that time (dates, flow, and symptoms) gives your doctor much more useful information than trying to recall patterns from memory. A simple period-tracking app or a calendar note is enough.