Irregular Periods: Common Causes, What to Watch For, and When to See a Clinician

What counts as irregular, and what is normal variation?

A typical menstrual cycle spans roughly 21 to 35 days, with bleeding lasting 2 to 7 days. Within those parameters there is significant individual variation — what is normal for one person may not be for another. Irregularity is best understood relative to your own baseline rather than a fixed textbook number.

Occasional variation — a cycle a few days early or late due to stress or illness — is usually not concerning. Patterns that suggest evaluation include:

• Cycles consistently shorter than 21 days or longer than 35 days
• Significant unpredictability, with cycles varying by 10 or more days
• Periods that stop entirely for three or more months (amenorrhea)
• Unusually heavy or light flow compared to your norm 1Ref 1American College of Obstetricians and Gynecologists (2019).Screening and Management of Bleeding Disorders in Adolescents With Heavy Menstrual Bleeding: ACOG Committee Opinion, Number 785.Heavy or prolonged menstrual bleeding as a feature of structural uterine causes (fibroids, polyps) and irregular periods warranting clinical evaluation
• Bleeding between expected periods (intermenstrual bleeding or spotting)

What are the most common causes of irregular periods?

Irregular periods usually reflect a disruption somewhere in the hormonal axis that regulates the cycle — the signaling between the hypothalamus, pituitary gland, and ovaries. Common causes include:

Stress and lifestyle factors. Physical or emotional stress can suppress the hormonal signals that trigger ovulation, causing delayed or missed periods. Significant weight change — both rapid loss and gain — affects hormone levels and can disrupt ovulation. Intense exercise, particularly when caloric intake is low relative to energy expenditure, can suppress menstruation through a condition called functional hypothalamic amenorrhea.

Polycystic ovary syndrome (PCOS). PCOS is among the most common hormonal conditions associated with irregular periods, involving excess androgens and often irregular or absent ovulation. It frequently goes undiagnosed for years 2Ref 2American College of Obstetricians and Gynecologists (2018).ACOG Practice Bulletin No. 194: Polycystic Ovary Syndrome.PCOS as a common hormonal cause of irregular or absent periods and androgen excess; evaluation approach including androgens and glucose.

Thyroid dysfunction. Both an overactive (hyperthyroid) and underactive (hypothyroid) thyroid can disrupt cycle regularity. Thyroid disease is one of the most important conditions to rule out, because it is common and easily treated once identified 3Ref 3Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Thyroid dysfunction as a common and easily treatable cause of irregular periods; TSH testing as part of evaluation.

Perimenopause. The transition to menopause, which typically spans the 40s, brings hormonal fluctuation that makes cycles increasingly unpredictable.

Hormonal contraceptives. Pills, patches, injections, IUDs, and implants all affect the cycle; irregular bleeding is common, especially in the first months 4Ref 4American College of Obstetricians and Gynecologists (2019).ACOG Practice Bulletin No. 206: Use of Hormonal Contraception in Women With Coexisting Medical Conditions.Hormonal contraceptives (pills, patches, injections, IUDs, implants) as a common cause of irregular or altered menstrual bleeding patterns.

Elevated prolactin (hyperprolactinemia). Sometimes caused by a benign pituitary growth, elevated prolactin suppresses ovulation.

Uterine structural causes. Fibroids or polyps can cause irregular or heavy bleeding even when ovulation is occurring normally 1Ref 1American College of Obstetricians and Gynecologists (2019).Screening and Management of Bleeding Disorders in Adolescents With Heavy Menstrual Bleeding: ACOG Committee Opinion, Number 785.Heavy or prolonged menstrual bleeding as a feature of structural uterine causes (fibroids, polyps) and irregular periods warranting clinical evaluation.

Are there less common causes worth knowing about?

Less frequently, irregular periods may signal conditions that warrant closer attention. Premature ovarian insufficiency (POI) occurs when the ovaries stop functioning normally before age 40 — uncommon but with implications for fertility and bone health. Certain chronic illnesses — including uncontrolled celiac disease, inflammatory bowel disease, and some autoimmune conditions — can affect cycle regularity. Endocrine conditions like Cushing's syndrome can disrupt the cycle. Some medications beyond contraceptives — including antipsychotics, certain antidepressants, corticosteroids, and chemotherapy — alter hormonal balance. This is why a thorough medication and health history is part of any irregular periods evaluation.

How should I track my cycle before seeing a clinician?

One of the most useful things to do before an appointment is to document your cycle pattern for as many recent cycles as you can recall — ideally two to three months of data. Cycle-tracking apps can help, but any written record works. Key things to note:

• The first day of each period (day 1 is the first day of actual flow, not spotting)
• How many days of bleeding
• Flow heaviness (light, moderate, heavy, or quantified by product use)
• Any spotting or bleeding between periods

This concrete picture is more useful to a clinician than a general impression of "irregular."

When should I see a clinician, and what will the evaluation involve?

A one-off late or early period rarely warrants urgent evaluation. Persistent irregularity — cycles that have been unpredictable for three or more months, periods that have stopped entirely, or a pattern distinctly different from your norm — is worth discussing with a clinician.

Most evaluations begin with a history and physical exam, then proceed to targeted lab work. Commonly considered tests include:

• TSH (thyroid-stimulating hormone) — rules out thyroid dysfunction, one of the most common and easily treatable hormonal causes 3Ref 3Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Thyroid dysfunction as a common and easily treatable cause of irregular periods; TSH testing as part of evaluation
• Serum or urine pregnancy test — always ruled out first
• Prolactin level — elevated prolactin suppresses ovulation
• FSH, LH, and estradiol — assess the hypothalamic-pituitary-ovarian axis
• Androgens (testosterone, DHEAS) — elevated levels support a diagnosis of PCOS 2Ref 2American College of Obstetricians and Gynecologists (2018).ACOG Practice Bulletin No. 194: Polycystic Ovary Syndrome.PCOS as a common hormonal cause of irregular or absent periods and androgen excess; evaluation approach including androgens and glucose
• Fasting glucose or insulin — insulin resistance is common in PCOS and affects treatment decisions
• Pelvic ultrasound — evaluates the uterus and ovaries for fibroids, polyps, ovarian cysts, or the characteristic appearance of PCOS

Treatment varies widely: lifestyle changes for stress- or weight-related irregularity, thyroid medication for thyroid disease, hormonal therapy for PCOS or perimenopause, and other approaches depending on the diagnosis.