According to MDEdge, many people believe that younger women and women who haven't had children have small uterine cavities and cervical diameters, and, as such, the IUD isn't appropriate for them. (businessinsider.com)
Then they will place a slender tube through the cervical opening and push the IUD through, into the uterine cavity — sort of like an extremely long and uncomfortable tampon applicator. (businessinsider.com)