The Math Behind Fertility Window Estimation

Ovulation prediction uses a simple statistical model: ovulation typically occurs 14 days before the next period starts. The fertile window spans 5 days before ovulation through 1 day after. The challenge is that cycle length varies. The calculation method The standard calendar method: Estimated ovulation = Next period start date - 14 days Fertile window start = Ovulation date - 5 days Fertile window end = Ovulation date + 1 day The "minus 14" comes from the luteal phase, which is the most consistent part of the menstrual cycle at 12-16 days (average 14). The follicular phase (before ovulation) varies more widely. function estimateOvulation ( lastPeriodDate , cycleLength ) { const nextPeriod = new Date ( lastPeriodDate ); nextPeriod . setDate ( nextPeriod . getDate () + cycleLength ); const ovulation = new Date ( nextPeriod ); ovulation . setDate ( ovulation . getDate () - 14 ); const fertileStart = new Date ( ovulation ); fertileStart . setDate ( fertileStart . getDate () - 5 ); const