Effectiveness should be a huge priority when using birth control! The depo shot is one of the most reliable methods of birth control available - of course nothing is 100% but when used as directed...
According to any info I've found, the Depo shot is 99.7% dependable but stranger things have happened than someone getting pregnant on it. I got my depo shots regularly for 5 years, never late, and...
Hopefully, you will not conceive while you are on depo, since it is a drug that you should not have while pregnant since it may cause birth defects. Wait until you have a good home situation before...