LH test (OPK)
- Input
- Ciclo irregular: 25–35 dias
- Expected output
- Pico de LH detectado 24–36h antes da ovulação
Most useful for identifying the fertile window in advance.
fertile window irregular cycle
Fertile Window with Irregular Cycles: What to Do
Irregular cycles make calendar calculation inaccurate, as ovulation can occur at highly variable times. Methods based on biological signs are more reliable in these cases.
Continue in the tool
Open full toolWhy does the calendar fail with irregular cycles?
- The calendar calculation assumes ovulation always occurs 14 days before the next period. In cycles that vary by more than 7 days between months, this assumption is unreliable.
- PCOS (polycystic ovary syndrome) is one of the most common causes of irregularity and can lead to anovulation (absent ovulation), making calculation useless.
- Recommended methods for irregular cycles: urinary LH tests (OPK), basal body temperature (BBT) and, if needed, follicular ultrasound.
Method comparison
Basal body temperature (BBT)
- Input
- Medição diária ao acordar
- Expected output
- Aumento de 0,2–0,5°C indica que ovulação já ocorreu
Confirms retroactively, does not predict.
Full tool FAQ
Ovulation occurs approximately 14 days before the next expected period, regardless of total cycle length. In 28-day cycles, that is around day 14. In 35-day cycles, around day 21. This calculation is an estimate for regular cycles; irregular cycles require complementary methods.
Frequently asked questions
How much can a cycle vary and still be considered "regular"?
Variations of up to 3 days between cycles are considered normal. Variations above 7 days indicate irregularity that warrants gynecological evaluation.