Carrying a pregnancy to term pretty much counts. Even if not dilated, the cervix effaces (shortens and flattens) during the last few weeks, and that means that even if you haven’t delivered vaginally things should be more amenable to stretching.
In case you don’t know- we’re talking about stretching the cervix to the diameter of the average ballpoint pen/drinking straw. The IUCD itself is about the size of a matchstick.
Most women who have had babies vaginally don’t require any dilation- you just sound the uterine cavity, insert the coil and Bob’s your uncle.
No babies mean you have to stretch things a little first (it all snaps back, don’t worry) but there shouldn’t be any major pushing or pulling.
If you are someone who feels faint or vomits during your period, you are more at risk of cervical shock. This basically means that your body is over sensitive to your cervix being dilated and your vagus nerve is over-stimulated so you get sick and woozy. During a coil insertion you might vomit, you might pass out, (very) worst case scenario, your heart could stop. Very rare, but possible. This is why inserting IUCDs in nulliparous women is riskier- bigger risks of uterine perforation, failure of insertion, expulsion of the device and cervical shock.
All doctors have their own comfort zone wrt risk, usually based on their own bad experiences. If your doc doesn’t do IUCDs in nulliparous women, it’s either because they personally had a bad experience, they saw something bad happen or they heard about something bad happening. I don’t really think you can blame them if they decide that they’ll minimise their own risk.
While I’ll be the first to tell you Mirena is an amazing invention and the person who came up with it deserves a medal, I won’t pretend that it is entirely risk free or that it suits everyone.