Depo Provera users are encourages to get a new injection every three months to ensure that the contraceptive is still active. According to the manufacturer, depo Provera works for "at least three months". So, it makes sense to have to get another shot after three months are up because that is the point at which it is possible that depo Provera no longer works. Nobody says that it is not possible for this method of birth control to be effective for much longer than that period of time, though.
Our readers have warned us that, in their experience, depo Provera can work a little too well, and be effective for up to two years. The simple answer is that depo Provera can indeed cause problems getting pregnant because it can prevent pregnancy for longer than you wanted. The hormones that normally mean depo Provera does its job well can stick around in your body long periods of time, and prevent you from ovulating. Of course, women who are anovulatory cannot get pregnant.
Perhaps the conclusion should be that depo Provera is not suitable for women who want to start trying to get pregnant in the next year or two. Depo Provera is more suitable for women who don't want a baby for a very long time, or so it seems! What if you are trying to conceive following depo Provera? The only possible suggestion is to keep at it the effects of the injectable birth control pill will, eventually, wear off.