A "regular" period doesn't mean your cycle has to be exactly 28 days. Cycle lengths between 21 and 35 days are considered clinically normal for adults, and some variation from cycle to cycle is expected.1
But when your cycle is consistently unpredictable — skipping months, arriving at wildly different intervals, or accompanied by unusual symptoms — it's worth understanding why.
What counts as irregular?
According to ACOG (the American College of Obstetricians and Gynecologists), a cycle may be considered irregular if:1
- Cycles are consistently shorter than 21 days or longer than 35 days
- Cycle length varies by more than 7–9 days from one cycle to the next
- Periods are absent for 90 days or more (amenorrhea)
- Bleeding lasts longer than 8 days
- Bleeding is significantly heavier or lighter than usual
Occasional irregular cycles — especially during times of stress, travel, or illness — are common and usually not concerning. The pattern matters more than any single cycle.
Common causes of irregular periods
1. Stress
Chronic stress activates the hypothalamic-pituitary-adrenal (HPA) axis, which can suppress the reproductive hormone cascade (GnRH → LH/FSH → estrogen/progesterone). This can delay ovulation, lengthen the follicular phase, or suppress ovulation entirely.2
The result: longer, shorter, or skipped cycles — depending on how the hormonal disruption plays out.
2. Polycystic ovary syndrome (PCOS)
PCOS is the most common endocrine disorder in reproductive-age women, affecting an estimated 8–13%.3 It's characterized by:
- Irregular or absent periods (often due to anovulation)
- Elevated androgen levels (which can cause acne, excess hair growth)
- Polycystic-appearing ovaries on ultrasound
PCOS is a leading cause of irregular periods and a common reason for difficulty conceiving. It's diagnosed based on the Rotterdam criteria (meeting 2 of the 3 features above).3
3. Thyroid dysfunction
Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can disrupt menstrual regularity:4
- Hypothyroidism tends to cause heavier, longer, or more frequent periods
- Hyperthyroidism tends to cause lighter, shorter, or less frequent periods
Thyroid disorders are common, treatable, and often missed. A simple blood test (TSH) can screen for them.
4. Significant weight changes
Both rapid weight loss and significant weight gain can affect cycle regularity:5
- Low body fat / caloric restriction: The hypothalamus may reduce GnRH secretion, leading to missed or absent periods (functional hypothalamic amenorrhea). This is common in athletes and people with restrictive eating patterns.
- Excess body fat: Adipose tissue produces estrogen, which can disrupt the hormonal balance and contribute to irregular cycles, often through anovulation.
5. Perimenopause
In the years before menopause (typically starting in the early-to-mid 40s, but sometimes earlier), cycles become increasingly irregular as ovarian reserve declines and hormone levels fluctuate unpredictably.6
Signs of perimenopause include:
- Cycles becoming shorter, then longer, then erratic
- Changes in flow (heavier or lighter)
- New or worsening PMS symptoms
- Hot flashes and sleep disruption
6. Hormonal contraception (starting or stopping)
Starting, stopping, or switching hormonal contraceptives can take 3–6 months to fully regulate. After stopping the pill, some people experience post-pill amenorrhea — delayed return of periods — which usually resolves within 3 months.7
7. Over-exercise
Excessive exercise without adequate caloric intake can suppress the HPG axis (hypothalamic-pituitary-gonadal), leading to irregular or absent periods. This is part of the Relative Energy Deficiency in Sport (RED-S) syndrome and is more common than many athletes realize.8
When to see a doctor
Schedule an appointment if:
- Your period has been absent for 3 or more months (and you're not pregnant or on hormonal contraception)
- Cycles are consistently shorter than 21 days or longer than 35 days
- You're soaking through a pad or tampon every hour for several consecutive hours
- You experience bleeding between periods that's new or increasing
- Irregularity is accompanied by excessive hair growth, severe acne, or rapid weight change (suggestive of PCOS or hormonal imbalance)
- You've been trying to conceive for 12 months without success (or 6 months if over 35)
What to bring to the appointment
The single most useful thing you can provide your doctor is accurate cycle tracking data — dates of periods, cycle lengths, symptom patterns, and any biometric trends (temperature, HRV) over multiple cycles. Prospective data is far more valuable than trying to recall dates from memory.
The bottom line
Irregular periods have many possible causes, ranging from benign (stress, travel) to clinically significant (PCOS, thyroid disorders). Tracking your cycles consistently is the first step toward identifying whether there's a pattern worth investigating.
References
- American College of Obstetricians and Gynecologists. Menstruation in girls and adolescents: using the menstrual cycle as a vital sign. ACOG Committee Opinion No. 651. 2015.
- Shors TJ, et al. Stress and the reproductive axis. Journal of Neuroendocrinology. 2001;13(6):508-515.
- Teede HJ, et al. Recommendations from the international evidence-based guideline for the assessment and management of polycystic ovary syndrome. Human Reproduction. 2018;33(9):1602-1618.
- Krassas GE, et al. Thyroid function and human reproductive health. Endocrine Reviews. 2010;31(5):702-755.
- Meczekalski B, et al. Functional hypothalamic amenorrhea and its influence on women's health. Journal of Endocrinological Investigation. 2014;37(11):1049-1056.
- Harlow SD, et al. Executive summary of the Stages of Reproductive Aging Workshop + 10. Journal of Clinical Endocrinology & Metabolism. 2012;97(4):1159-1168.
- Girum T, Wasie A. Return of fertility after discontinuation of contraception: a systematic review and meta-analysis. Contraception and Reproductive Medicine. 2018;3:14.
- Mountjoy M, et al. IOC consensus statement on relative energy deficiency in sport (RED-S). British Journal of Sports Medicine. 2018;52(11):687-697.