Hello, and I hope I can help you today.
If the depo shot is given to you in the first five days of your menstrual cycle, or if you had a negative pregnancy
test at the time the shot was given, it is immediately effective in preventing pregnancy. So you do not need to use a condom or other backup method if you are having unprotected sex to prevent pregnancy while using the Depo-Provera
Depo-Provera in addition causes irregular to absent menstrual cycles. This will continue as long as you use the shot, however you are protected against pregnancy as long as you get your subsequent injections on time. Many women enjoy the convenience of not bleeding every month.
So to ease your peace of mind, you may perform a urine pregnancy test, but if it is negative you really do not need to be concerned about accidental pregnancy as the shot is nearly 100% effective. You should continue to use condoms, if desired, to protect from STDs.
I hope that I was able to adequately answer your question today, and that this information was reassuring.