What are the chances of pregnancy when you are on the IUD Mirena?

Mirena is 99.9% effective; 1 in 1,000 women can experience a pregnancy while having the Mirena. This IUC is highly effective but like with any bc method, there's always the possibility of becoming pregnant, as minimal as it can be.

Not totally unheard of but I would say pretty unlikely one of the better forms of contraceptive. But like all contraceptives this one is only 99% effective.