How do copper IUDs 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.

Copper intrauterine devices (IUDs) prevent pregnancy primarily by releasing copper ions into the uterus. These ions impair the viability of sperm, which effectively prevents the fertilization of the egg.1

Unlike hormonal IUDs, which rely on synthetic hormones to thicken cervical mucus or suppress ovulation, the copper IUD functions as a non-hormonal form of birth control that provides immediate protection upon insertion.1