Fertile window
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Estimated ovulation day—
Window length6 days
Estimate which days of your cycle fall in the fertile window, based on your average cycle length.
Ovulation typically occurs about 14 days before the next period starts, so it falls on cycle day (cycle length − 14) — this shifts earlier or later depending on your personal cycle length. The fertile window itself — about 5 days before ovulation through the day of ovulation — stays roughly the same length regardless of cycle length, since sperm survival time doesn't change; what changes is where that window falls within your cycle.
ovulation day = cycle length − 14 · fertile window = (ovulation day − 5) through ovulation day