This is an updated guideline from the FSRH. There isn’t really anything very new in this, so I’ve just summarised some of the things I wasn’t aware of or was a bit hazy on.
Method of action
Copper intrauterine devices (Cu-IUD) provide a hostile environment to fertilisation, have a cervical mucous effect and an anti-implantation effect.
Levonorgestrel Intrauterine Systems (LNG-IUS) like Mirena and Jaydess (and now Levosert) have an uncertain method of action. They seem to work through cervical mucous and by effects on the endometrium. Some women stop ovulating with an LNG-IUS.