Hi there,
Depo vera is injected every 12 weeks while the contraceptive has the power to prevent conception up to 14 weeks, the injections are usually given every 12 weeks to ensure that there is no gap in the effectiveness of the birth control between shots. If such a schedule
is maintained, pregnancy while on
Depo-
Provera is extremely rare. Since your girlfriend was not on taking it regularly there might have been a period when she had sex while not on depo.