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The days when conception is possible are the days just before and including the day of ovulation. Based on the MAXIMUM lifespan of human sperm and ova, (five days for sperm and one day for the ovum) this fertile window is at MOST six days. This MAXIMUM window is thus made up of the five days before ovulation and the day of ovulation.
Though pregnancy is technically possible during this six day window, most pregnancies actually can be attributed to intercourse from within TWO DAYS before ovulation up to the day of ovulation.
This makes for a practical fertile window of just THREE DAYS each cycle: the TWO DAYS before ovulation and the DAY OF OVULATION itself.
:o