The time of ovulation has great practical importance because the possibility of fertilization-the fusion of a sperm and egg-can occur only during a short period of time during each menstrual cycle. Although a few "super" sperm may remain viable for up to 5 days, most sperm retain their fertilizing power for only 24 to 72 hours after being deposited in the female reproductive tract following ejaculation. And, the oocyte remains viable and capable of being fertilized for only about 12 to 24 hours after ovulation. A woman's fertile period therefore lasts only a few days each month-from between 3 to 5 days before, and on later than 24 hours after ovulation.