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Fertility
Conception Timing Tool
Cycle-based fertile window calculator with daily probability per intercourse timing. Predicts ovulation across the next three cycles and outputs timing strategy.
Cycle Inputs
Day 1 is the first day of full menstrual flow. Default cycle length is 28 days with a 14-day luteal phase. Adjust based on cycle tracking history.
Day 1 of the most recent cycle.
Day 1 to next day 1. Most cycles: 24-35 days.
Typically 12-14 days. Most stable cycle component.
How fertile window is calculated
Ovulation occurs approximately (cycle length minus luteal phase) days after the start of menses. For a 28-day cycle with 14-day luteal phase, that is day 14. Sperm survive in cervical mucus up to 5 days. The egg is viable for roughly 12-24 hours after ovulation. The fertile window is the 5 days before ovulation plus the ovulation day itself.