My last Depo shot had lapsed a couple of months, as a precautionary measure, my doctor ran a pregnancy test prior to giving the new shot. When she came back in, she gave no explanation of the test results. When I asked about it she explained that the reason she hadn't thought to tell me, was because the likelihood of someone who has been on Depo for even a year getting pregnant is VERY, VERY, slim. As I've been on it for almost 2yrs now, & for over 3 years prior, she said she hadn't been very concerned about pregnancy in my case; that a test was just standard procedure when any B.C. lapses.
To make a long story short, I was told by a medical professional that it is difficult for women who are actively TRYING to get pregnant to succeed when they've used Depo for a moderate lenghth of time. In fact, she said that it usually takes 2-3 months off Depo, for every month on, to conceive effectively!
There are always exceptions, & it is (of course) ALWAYS possible to get pregnant while on ANY birth control, but having a doctor tell me SHE wasn't worried was really reassuring. If you WANT to get pregnant, sorry to be a nay-sayer... If you DON'T, hope this helped give you a bit of perspective... although, I would talk to my own doctor about it too, if I were you!!