Depo Provera is extremely effective at preventing pregnancy for "at least three months". When you have stopped getting the injections, and would like to try to conceive, what that essentially means is that it might well take a lot longer for you to start ovulating again (and thus be fertile) than with many other forms of birth control, including the birth control pill and a Mirena coil. It is not possible to predict how long depo Provera will take to wear off before stopping the injections. For some, it takes three to six months. Others have less luck, and do not start ovulating again until two years after receiving their final shot!
What does this mean? One can only conclude that, if you are hoping to get pregnant in the relatively near future but want to current prevent conception, depo Provera is not the right birth control for you. Unfortunately, the fact that ovulation may be halted long after a woman stopped getting depo Provera injections is not very well publicized. This is one huge disadvantage of this form of birth control! Contraceptives that might be more suitable for women who want to get pregnant in the next two years include the birth control pill, condoms, an intrauterine device (AKA coil), and charting to avoid.