Mirena iud? risks?

i was ust wondering is anyone could tell me what the risks of getting pregnant on the mirena iud. not the rates, just what happens if you were by chance to get pregnant while on it, to the baby. would you automatically miscarriage? is it dangerous for the baby? any info would help, im considering getting it but if i do get pregnant, i would like to know how it could affect an unborn baby. thanks!