Lorien summed it up pretty well. It's something you need to talk to your gyno about, and if he/she/you is worried, or if it's really bothering you, another birth control method might be best.
While Depo is 99.7% effective, it's one of the strongest methods out there. Lorien mentioned it can cause irreversible bone density loss when used over the 2 year mark, and it has other serious and potentially irreversible side effects. You might want to check out the Depo Sticky to make sure you fully understand what you're putting in your body and what the potential risks and side effects are:
http://www.gaiaonline.com/guilds/viewtopic.php?t=4293323