Mirena Insertion

Generally all intra-uterine devices are inserted as an in-rooms procedure.
If you have

then the IUD may be too difficult to insert and a general anaesthetic may be required.

The risk of uterine perforation and infection is low, both of these risks occurring at the time of insertion (whether in the rooms or theatre).
A risk of trying to pass the 6mm diameter introducer through the cervix is a profound vaso-vagal episode. This is a profound feint. Your blood pressure and pulse rate fall dramatically; you can pass out and even have a fit. This is unusual but can occur.