If that lining isn't strong enough then it's possible to become pregnant.
Also they say you shouldn't get for more than 2 years. Our bodies do tend to become immune to things and as a result things end up not working as effectively.
That lining is what is supposed to prevent sperm from meeting an egg and from your body releasing an egg.
So don't listen to anyone who says they've gotten pregnant on the shot.
No, you've gotten pregnant on the shot because you don't use it right.
Now stop saying people aren't using it right its not like you're taking the pill.
I know this is an old post but I wanted to give my two cents.