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At the top of the vagina is the cervix, which is the entrance to the uterus. The cervix is actually the lower portion of the uterus, and it protrudes approximately one-third of an inch into the vagina. The cervix produces a special mucus that changes according to the woman’s menstrual cycle — the monthly process of releasing eggs in preparation for possible pregnancy (see “The ‘Men’ Words: Menstruation and Menopause,” later in this chapter).

 During the first half of the cycle, especially around the time the woman releases the egg (ovulates), her mucus is abundant, clear, and watery. It is quite receptive to sperm penetration and survival, and sperm may live in the mucus for several days.

 After ovulation occurs, the cervical mucus changes dramatically. It becomes thick, cloudy, and sticky, just about impenetrable to sperm trying to pass through the os, or opening of the cervix.

Making the cervical mucus impenetrable to sperm is one of the ways that the birth control pill prevents pregnancy. In addition, those practicing natural family planning can test the quality of the mucus as an indicator of fertility (see Chapter 7 for more information on this technique).