How an IUD Prevents Pregnancy
There are 4 main ways:
Thickening of the cervical mucus, which prevents sperm from entering the uterus.
Inhibiting sperm movement so it’s more difficult to reach and fertilize an egg.
Making the lining of the uterus that builds up before your period thinner, which makes it less likely for an egg to attach to the uterus.
Preventing the release of eggs from the ovaries (hormonal IUDs only).
So through all of these methods, an IUD prevents pregnancy by not allowing the sperm to ever meet the egg and fertilize it in the first place!