Fertilisation is a complex process that must be accomplished within a strict time frame. Theoretically, a man is always fertile, but the woman's eggs can only be fertilised within a specific 12 - 24 hour period shortly after ovulation. Therefore, there is only a "window of opportunity" between 24 - 48 hours each month when intercourse can be expected to result in fertilisation. Timing is critical if the egg and sperm are to survive the journey through the woman's reproductive tract, unite, become fertilised, and result in the embryo implanting successfully into the uterine wall.

The man deposits between 100 and 200 million sperm into the woman's vagina with each ejaculation of sperm. During normal intercourse, or even after the woman has been artificially inseminated, much of the semen pools in the vagina behind the protruding cervix. Because the cervix usually points partially backward into the vagina, the cervix is usually immersed in the pool of ejaculated semen. This emersion helps direct the sperm through the cervix and into the reproductive tract. Muscular contractions in the fallopian tubes help the actively swimming sperm to reach the egg in the mid portion of the fallopian tube where fertilisation takes place.