You can also remind her that the IUD is not absolute.
Some of us here were conceived and born while their mothers were using an IUD

While I was developing, the IUD became loose and became lodged in tissue. Due to the fact that ultrasound was not yet that good in 1974, they had no clue where. Luckily for me, it was in the placenta…and had no long term effects.
Ask her what if she get’s pregnant, and the IUD lodges in the new babies brain? Or leg? Or heart? Or any other part of the body.
In any event, I feel your pain because my wife is the same way. She wants to use ABC, and even have ME use it to prevent pregnancy. There have been our deepest fights about it. She’s pregnant now, so it’s not an issue right now. But I know it will be soon. We just didn’t have relations, and that was more strain…but it was a cross to bear for being obedient to Church teaching.
I suggest talking to a Priest/your confessor/spiritual director. They know more about your specific situation to guide you.
Andy