If you have sex, you can get pregnant no matter what you're using! NO method of birth control is 100% effective except not having sex, but Depo Provera is 99.7% effective, meaning that out of 1000 women using Depo for one year, three would get pregnant through no fault of their own. This is a comparable failure rate to the pill taken perfectly, the patch, the ring, the IUD, and permanent sterilization.
I've been using depo as my only form of bc for three years and haven't had any problems at all. The best thing about it, is there is no human error factor (provided you get the shot on time every three months), so it's actually considered more effective than things like the pill. Also, after you stop it it USUALLY takes at least 6 months (or more) to get pregnant.
Co-cl for Birth Control