Is there such thing as bad sperm?

Okay i know there no such thing as bad sperm if it already impregnated a women but I've been with two different girls and the first one had two miscarriages on me and now my current girlfriend has had a miscarriage and is currently pregnant at the moment. If this pregnancy doesn't go through I'm going to be really devastated. So my question is, am i the problem here? I have an athletic build, I don't smoke, i do drink occasionally. I don't do drugs and I'm 27 years old.