Many times dating is a big issue, as well as making sure the mother knows if she is pg or not. And of
course if possible they would like to prevent giving BC when pg. But Depo
works to trick your body into feeling it is pg by the high dose of
synthetic progesterone which suppresses ovulation (which pg does anyway) and thickens cervical mucus (which pg does as well).
It does not appear that there are any significant problems with having a shot and then finding out you are pregnant.