Can the shot stop you from having kids after it is out of your system?

i had been on the depo shot for three years and became pregnant and didnt no cause you don't have a period on the shot. so i went for my next shot and found i was 3 months pregnant. my daughter was born premature at 32 weeks and only weighed 2lb11oz. so i belive if your meant to hav a child at a certain time you will no matter what Birth Control your on, it will happen when its supposed to...