How does a cervical cap work in the body?

Cervical cap. The cervical cap is a form of barrier contraception. It is inserted by the women and, as the name implies, it sits like a "cap" over the cervix. This prevents the sperm from entering the uterus & thereby preventing fertilization of the egg. Spermicide is also added to the dome portion of the cap to increase the effectiveness. Your doctor will need to help select the right size for you.