Find the six days each cycle when conception is most likely.
28-day cycle, last period 1 May
Seven days spanning ovulation around 15 May.
Fertile 10–16 May
32-day cycle
Ovulation moves to about day 18, and the window follows.
Window shifts later
How the figures above are worked out — the rules, rates and reasoning behind every line.
The fertile window is the small number of days each cycle when intercourse can lead to pregnancy. This pinpoints those days from your last period and cycle length.
Sperm can survive in the body for up to about five days and the egg lives roughly 24 hours, so the window runs from five days before ovulation through ovulation day.
Ovulation is estimated as (cycle length − 14) days after your last period started.
About seven days: the five days before ovulation, ovulation day, and the day after. The two days before ovulation are usually the most fertile.
It's much less likely, but cycles vary and sperm can survive several days, so the window is a guide rather than a guarantee.
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