Overview.
Irregular periods — cycles outside the 21-35 day range, with significant variability, or absent entirely — nearly always reflect a hormonal or structural abnormality when persistent. A targeted lab workup identifies the cause in the majority of cases. Pregnancy must always be excluded first.
Oligomenorrhea: cycles >35 days or <8 per year. Primary amenorrhea: no menstruation by age 15 (or 13 without secondary sexual characteristics). Secondary amenorrhea: no periods for ≥3 months in previously regular women, or ≥6 months if previously irregular. Normal cycles: 21-35 days, flow 2-7 days.
Prevalence: Up to 15-20% of reproductive-age women experience oligomenorrhea. Secondary amenorrhea (excluding pregnancy) affects ~3-5% of women at any given time.
What to test.
First-line tests
- Pregnancy test (urine or serum hCG) — Always first — never skip
- FSH — Elevated >25 IU/L suggests ovarian insufficiency or perimenopause
- LH — Elevated LH:FSH ratio supports PCOS; low LH + low FSH suggests hypothalamic cause
- Prolactin — Elevated >20-25 ng/mL indicates hyperprolactinemia
- TSH — Both hypo and hyperthyroidism cause menstrual irregularity; always exclude
- Estradiol — Low E2 + high FSH = ovarian failure; low E2 + low FSH = hypothalamic amenorrhea
- Total Testosterone + SHBG — Elevated in PCOS; markedly elevated suggests adrenal/ovarian tumor
Second-line tests
- AMH — POI or DOR workup; also elevated in PCOS
- 17-OH Progesterone — Early morning follicular phase; >2 ng/mL suggests non-classic CAH
- DHEA-S — >700 mcg/dL warrants adrenal imaging
- Fasting Glucose + Insulin / HOMA-IR — If PCOS confirmed or metabolic cause suspected
- Cortisol (1mg DST or 24h urine) — If Cushing's syndrome suspected
Common causes.
- PCOS (Most common cause of oligomenorrhea in reproductive-age women)
- Functional Hypothalamic Amenorrhea (Common in athletes, underweight women, high-stress situations)
- Hyperprolactinemia (Pituitary microadenoma, medications, or hypothyroidism)
- Hypothyroidism (Common; always exclude with TSH)
- Premature Ovarian Insufficiency (POI) (Less common; urgent workup needed)
- Perimenopause (Physiological in women 45+)
- Non-classic Congenital Adrenal Hyperplasia (Less common; check 17-OH progesterone)
- Asherman's Syndrome (Uterine adhesions from prior instrumentation)
FAQs.
How irregular is too irregular?
Cycles consistently shorter than 21 days or longer than 35 days, or that vary by more than 7-9 days in length, are considered irregular. Occasional irregularity (one or two unusual cycles per year) is generally normal. Persistent irregularity over 3+ cycles warrants evaluation.
Can stress cause irregular periods?
Yes. Psychological and physiological stress (excessive exercise, caloric restriction, illness, emotional stress) can suppress the hypothalamic-pituitary axis, causing functional hypothalamic amenorrhea. This is a diagnosis of exclusion — other causes must first be ruled out.
Does irregular periods mean I am infertile?
Not necessarily. Many causes of irregular periods are treatable (PCOS, hyperprolactinemia, thyroid disease), and fertility often restores with appropriate management. The significance depends on the underlying cause and whether ovulation is occurring.