To the Editor: In the discussion of biologic differences between male and female fertility (April 6 issue),1 Federman states that women are fertile for only 12 hours each month. Although the egg is viable for 12 hours or less, the window of fertility in women is approximately five to six days in each menstrual cycle,24 depending on the presence of estrogenic cervical mucus that maximizes the storage, survival, and transport of sperm until ovulation.4,5 Dr. Federman replies: Stanford rightly calls attention to the elegant estrogen-dominated events that precede ovulation and that favor passage of sperm through the cervix . . .