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How do diaphragms and cervical caps prevent pregnancy?

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Blister pack of 28 oral contraceptive pills arranged in four rows, with 21 white active pills followed by 7 orange inactive pills. Arrows indicate the order for taking pills. — birth control A 28-day package of birth control pills.

Diaphragms and cervical caps are barrier devices that prevent pregnancy by physically blocking sperm from entering the uterus.1

A cervical cap is a thimble-shaped device that fits snugly over the uterine cervix to create a barrier. Similarly, a diaphragm covers the cervix to obstruct the passage of sperm. To be effective, both devices must be used in conjunction with a spermicide, such as a cream or jelly. The spermicide serves to kill sperm and prevent them from bypassing the barrier, which significantly improves the device's overall effectiveness.21

While these methods are among the safest forms of contraception due to their lack of systemic health risks, their success in practice depends heavily on the user's ability to use them consistently and correctly.1