When Does Mirena Coil Get Activated After Its Insertion?

hi there, my girlfriend has her appointment to have her mirena inserted this coming friday however her period just ended. i have read they generally insert it during the time she is having her period however i have read it can be inserted after as well as her doctor explained to her but i have read conflicting information on weather we have to wait a week before it is active or if it is still active right away. any information would be very helpful.