According to NHS Choices, an ovum, or egg, is only viable for 12 to 24 hours after it moves from the ovaries. Sperm, on the other hand, can stay alive in the woman's fallopian tubes for up to a week, notes the site.
The long viability of sperm gives couples a greater window for fertilization, as NHS Choices states. Ovulation generally occurs about two weeks before a period though it depends on the person. FertilityFriend.com notes that the day of ovulation plus the five days leading up to ovulation is the window of time during which conception can occur. Having intercourse two days prior to ovulation offers the best chance that conception occurs.