Why Is My Period Late? Common Causes and What to Do Next

A late period (medically termed oligomenorrhea or amenorrhea) is incredibly common and rarely follows a single predictable pattern. Your menstrual cycle acts as a sensitive biomarker of overall physiological status, responding to everything from stress to hormonal shifts.

Common Causes of a Late Period

• Pregnancy: The number one cause in reproductive-age women. Test if you're sexually active.
• Stress (physical or psychological): Activation of the HPA axis suppresses GnRH, leading to delayed or absent ovulation.
• Weight changes: Rapid loss or gain, low BMI (<18.5), or high BMI (>30) disrupt leptin and insulin signaling.
• Exercise: High volume/intensity combined with low energy availability can cause functional hypothalamic amenorrhea.
• Hormonal contraception: Combined pills dictate withdrawal bleed timing; progestin-only methods often cause irregular or absent bleeding.
• Thyroid dysfunction: Both hypothyroidism and hyperthyroidism can alter cycles.
• Elevated prolactin: From prolactinomas or medications like antipsychotics and metoclopramide.
• PCOS: Polycystic ovary syndrome causes oligo- or anovulation with hyperandrogenism.
• Perimenopause: Erratic cycles typically begin in the 40s.
• Medications: Antidepressants, antipsychotics, chemotherapy, glucocorticoids.
• Acute or chronic illness: Infection, surgery, travel, circadian disruption.

Does One Late Period Predict the Next?

Not reliably. The next cycle depends on whether the underlying cause persists or resolves. If a temporary stressor is removed, ovulation often resumes on schedule. If the cause is chronic (PCOS, thyroid disorder, weight issues), the pattern may continue.

When to Test or See a Clinician

• Home pregnancy test: On day 1 of a missed period (or 21 days after unprotected sex). Repeat in 48 hours if negative but still no period.
• See a clinician if:
  • More than 90 days without a period (not pregnant, not on continuous hormonal contraception)
  • Fewer than 3 cycles per year
  • Heavy or prolonged bleeding when it arrives
  • Symptoms like hirsutism, acne, weight change, hot flashes, or galactorrhea
  • Trying to conceive for >12 months (<35 years) or >6 months (≥35 years)

Typical Diagnostic Workup

1. hCG (urine or serum)
2. TSH, prolactin, FSH, LH, estradiol, testosterone, DHEA-S, 17-OHP
3. Pelvic ultrasound (ovarian morphology, endometrial thickness)
4. Glucose, insulin, lipids if PCOS is suspected

Reassurance: One late period is usually a transient adaptation. Persistent irregularity signals a treatable underlying condition.