Most of the spermatozoa in the ejaculate are contained in the very first portion of fluid that squirts out of the penis and enters the vagina. The remaining squirts usually contain very little sperm. Thus, at the first moment of ejaculation the female’s cervix (the opening leading into her uterus) is bathed by a high concentration of sperm. Within just a few minutes after ejaculation, sperm begin to invade a very thick fluid (called cervical mucus) that is pouring out of the cervix. The sperm must be able to invade the cervix via the cervical mucus by virtue of their own swimming ability. Nothing about the sexual act will help those sperm get into the cervix. They simply have to swim into the mucus on their own, and this requires a great deal of coordinated, cooperative activity on their part.

Ejaculation is a challenging moment for the sperm, as the vagina presents a very harsh, acidic environment, which would normally immobilize them quickly. The alkalinity of the semen (the fluid that contains the sperm), as well as the alkalinity of the cervical mucus, allows the sperm to survive in this difficult vaginal milieu. Any acidity at all quickly kills sperm.

Yet even the semen is a potentially dangerous milieu for the sperm; any sperm that remain in the semen for more than two hours are likely to deteriorate. In order to survive long enough to get to the egg and fertilize it, the sperm must gain rapid access to the cervical mucus. Any sperm that have not penetrated the cervical mucus within a half hour after orgasm will not be able to do so later on, because by then they will have lost their ability to swim into the more friendly environment of the cervix.