A diaphragm is a dome-shaped device that is inserted into the vagina prior to intercourse as a contraceptive method. The device is typically made of silicone and has a flexible rim. It is designed to cover the cervix to prevent sperm from entering the uterus.
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A diaphragm is designed to prevent pregnancy because it covers the cervix, thus the opening of the uterus during intercourse. For a diaphragm to be as effective as possible it should be used with spermicidal lube, jelly, or foam. The spermicide will stop the sperm from moving. As with most forms of birth control, even when used correctly, a diaphragm is not 100% effective, and should be used in conjunction with another form of birth control.