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- Enter the first day of your last period.
- Enter your average cycle length (from the first day of one period to the first day of the next).
- Optionally adjust the luteal phase length if you know it (typically 12–16 days, average 14).
Formel
Ovulation usually happens about one luteal phase before the next period. The fertile window covers the five days before ovulation, because sperm can survive up to five days, plus the day after.
Ovulation ≈ LMP + cycle length − luteal phase
Fertile window = ovulation − 5 days … ovulation + 1 day
Example
- LMP on 1 June, 28-day cycle, 14-day luteal phase.
- Ovulation ≈ 15 June; fertile window 10–16 June.
- Next period expected 29 June.
Häufige Fragen
How reliable is the calendar method?
It is an estimate. Cycles vary from month to month, especially if they're irregular. Ovulation tests (LH strips), basal body temperature and cervical mucus tracking are more precise.
Can I use this to avoid pregnancy?
No. Calendar-based predictions are not reliable contraception. Talk to a healthcare provider about effective birth-control methods.
When should I take a pregnancy test?
Most home tests are reliable from the first day of a missed period. Testing earlier can give a false negative.