What Causes Irregular Periods, and When Should You See a Doctor?

A "regular" period does not have to follow a 28-day cycle. For adults, cycle lengths between 21 and 35 days are considered clinically normal, and some variation between cycles is expected.

If periods repeatedly skip months, arrive at very different intervals, or come with unusual symptoms, the pattern is worth looking into.

What counts as irregular?

According to ACOG (the American College of Obstetricians and Gynecologists), a cycle may be considered irregular if:

• 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

An occasional irregular cycle is common, especially during stress, travel, or illness, and is usually not concerning. A repeated pattern matters more than one cycle.

Common causes of irregular periods

1. Stress

Chronic stress activates the hypothalamic-pituitary-adrenal (HPA) axis. This can suppress the reproductive hormone cascade (GnRH → LH/FSH → estrogen/progesterone), delay ovulation, lengthen the follicular phase, or suppress ovulation entirely.

Depending on how the hormonal disruption develops, cycles may become longer or shorter, or a period may be skipped.

2. Polycystic ovary syndrome (PCOS)

PCOS is the most common endocrine disorder in reproductive-age women and affects an estimated 8–13%. Its features include:

• 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. Diagnosis is based on the Rotterdam criteria, which require 2 of the 3 features above.

3. Thyroid dysfunction

Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can disrupt menstrual regularity:

• 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:

• With low body fat or caloric restriction, the hypothalamus may reduce GnRH secretion. This can lead to missed or absent periods (functional hypothalamic amenorrhea) and is common in athletes and people with restrictive eating patterns.
• With excess body fat, estrogen produced by adipose tissue can disrupt hormonal balance and contribute to irregular cycles, often through anovulation.

5. Perimenopause

In the years before menopause, cycles often become more irregular as ovarian reserve declines and hormone levels fluctuate unpredictably. This typically starts in the early-to-mid 40s, though it can begin earlier.

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)

After starting, stopping, or switching hormonal contraceptives, cycles can take 3–6 months to fully regulate. Some people experience post-pill amenorrhea, or a delayed return of periods, after stopping the pill. This usually resolves within 3 months.

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 Relative Energy Deficiency in Sport (RED-S) syndrome and is more common than many athletes realize.

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

Accurate cycle tracking data can be useful at the appointment. Bring dates of periods, cycle lengths, symptom patterns, and any biometric trends (temperature, HRV) from several cycles. Prospective data is more useful than trying to recall dates from memory.

When irregularity becomes a pattern

Irregular periods can have relatively benign causes, such as stress or travel, as well as clinically significant causes, including PCOS and thyroid disorders. Consistent cycle tracking can show whether a pattern is developing and give a doctor more specific information to evaluate.

---

References

1. 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.
2. Shors TJ, et al. Stress and the reproductive axis. Journal of Neuroendocrinology. 2001;13(6):508-515.
3. 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.
4. Krassas GE, et al. Thyroid function and human reproductive health. Endocrine Reviews. 2010;31(5):702-755.
5. Meczekalski B, et al. Functional hypothalamic amenorrhea and its influence on women's health. Journal of Endocrinological Investigation. 2014;37(11):1049-1056.
6. Harlow SD, et al. Executive summary of the Stages of Reproductive Aging Workshop + 10. Journal of Clinical Endocrinology & Metabolism. 2012;97(4):1159-1168.
7. Girum T, Wasie A. Return of fertility after discontinuation of contraception: a systematic review and meta-analysis. Contraception and Reproductive Medicine. 2018;3:14.
8. 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.