How badly do your testicles need to get hit to prevent childbirth?

No one answer: The answer is not how hard, but the damage done. I've seen cases where the injury was actually very mild, but hit an important blood supply just right or caused a testicle to twist (called torsion) and there was loss of function. On the other hand, i've seen some pretty swollen, bruised testicles with a lot of acute injury that went on to heal fine and remained functional.