Progesterone-based IUDs primarily work by thinning the uterine lining and do not always prevent ovulation.

“for these work mostly by thinning out the uterine lining as we already said progesterone compacts the uterine lining to prepare it for implantation in a normal cycle but if you have constant exposure to Progesterone what is going to happen is it's going to prevent the uterine lining from growing and it gets it very very thin not all iuds in fact most of them don't prevent ovulation only in about 50% of people do they actually prevent ovulation so their main mechanism of action is this endometrial effect”