Depo shot and pregnancy try-out?

i was wounding if you can draw from a false negitive when on the depo shot. i was told you do and also told you dont plz back me! I was on depo and still get pregnant.
The real grill would be do you get your shots when you are supposed to. Depo prevents ovulation and alters teh pool liner of your uterus so an egg can't implant.