A diaphragm is a thin rubber dome with a springy and flexible rim. It is inserted into the vagina, fits over the cervix and is held in place by vaginal muscles. A diaphragm holds spermicide in place over the opening to the uterus. Spermicide kills sperm, preventing fertilization. After intercourse, it should be left in place for 6-8 hours. Diaphragms are 86-94% effective as birth control. They offer limited protection against sexually transmitted infections, but do not protect against HIV/AIDS.