Irregular periods (cycles shorter than 21 days or longer than 35 days) often indicate irregular or absent ovulation, which is the most common cause of female infertility. But irregular does not mean infertile. The most common cause — PCOS — is highly treatable. Other causes include thyroid dysfunction, stress, low body weight, and perimenopause.
What Counts as Irregular
- Normal: Cycles between 21–35 days, varying by up to 7 days cycle to cycle
- Irregular: Cycles consistently shorter than 21 or longer than 35 days, or varying by more than 7–9 days
- Absent (amenorrhea): No period for 3+ months (if not pregnant or breastfeeding)
Common Causes
| Cause | How Common | Key Sign | Treatment |
|---|---|---|---|
| PCOS | 8–13% of women | Long cycles (35–90+ days), acne, excess hair growth | Letrozole for ovulation induction; inositol; weight management |
| Thyroid dysfunction | 5–10% of women | Fatigue, weight changes, temperature sensitivity | Levothyroxine (hypo) or antithyroid medication (hyper) |
| Hypothalamic amenorrhea | Common in athletes and underweight women | Absent or very light periods; low BMI; high exercise | Increase caloric intake; reduce exercise intensity; may need medication |
| Hyperprolactinemia | 2–5% of women | Milky nipple discharge; irregular cycles | Cabergoline or bromocriptine to lower prolactin |
| Perimenopause | Typically starts in early-to-mid 40s | Cycles becoming shorter then irregular; hot flashes | Evaluation of ovarian reserve; may need fertility treatment |
| Stress | Very common but often temporary | Period delayed during high-stress periods | Stress management; usually self-resolving |
How to Track Ovulation With Irregular Cycles
Calendar-based methods are useless with irregular cycles. You need biological signals:
Tracking strategies for irregular cycles
- OPKs: Start testing early (day 8–10) and test daily until you get a surge. Budget for more strips per cycle.
- Cervical mucus: Watch for egg-white mucus as your primary fertility cue — free and always available.
- Quantitative monitors (Mira, Inito): Especially useful for PCOS patients where standard OPKs can give false positives due to chronically elevated LH.
- BBT: Good for confirming ovulation occurred, even if you can't predict when it will happen.
Getting Pregnant With PCOS
PCOS is the most common cause of irregular periods. Our complete guide covers treatment pathways.
Read the PCOS Guide