For sex to result in a pregnancy, a woman has to go through ovulation. Part of the monthly menstrual cycle, ovulation happens when an egg, or ovum, becomes available for fertilization by a sperm cell.
During ovulation, an egg moves from the ovaries (the organs that produce eggs) into the fallopian tubes. There, a sperm cell can fertilize the egg, which could then move into the uterus, or womb, and develop into a fetus.
During ovulation, the walls of the uterus also thicken to prepare for a fertilized egg. But if the egg is not fertilized, the uterus sheds that lining, causing the monthly bleeding of a menstrual period.
Many people mistakenly believe ovulation always happens exactly 14 days after a woman's last period. But the timing varies for each woman, usually falling between days 11 and 21 of the menstrual cycle.
Mammals other than humans and apes go through an "estrous cycle" instead of a menstrual cycle. In this cycle, females are only sexually active during their estrous phase — sometimes called "being in heat." Both human and ape females can be sexually active at any time in their cycles. However, mammals with estrous cycles don't have menstrual periods, because the uterus reabsorbs its lining instead of shedding it.