Reproductive System

The vagina is a muscular tube about 5 in (12.7 cm) long. A thin layer of tissue called the hymen may cover the vaginal opening, but is usually gone in physically or sexually active females. A mucous membrane lines and moistens the vagina. During sexual intercourse, the vagina is lubricated further and functions to direct the penis toward the cervix to optimize fertilization. During childbirth, the vagina stretches to accommodate the passage of the baby. Both the uterus and the vagina contract to relatively original sizes some time after delivery.

Some contraceptive devices act as a barrier between semen and the vagina or semen and the cervix. A condom placed correctly on a man's penis can prevent sperm from entering the vagina. A diaphragm is a rubber, cup-shaped contraceptive inserted into the vagina prior to intercourse that acts as a physical barrier between semen and the cervix; it is usually used along with a spermicidal jelly to chemically kill sperm. Other contraceptives, such as the birth control pill and depo-provera usually inhibit the function of progesterone to prevent ovulation.