I have a problem. When I was a little boy I knock one of my testicles against the frame of my bike. Since then my left testicle is smaller than the other. I've been wondering why my girlfriend had a miscarriage. Could it be because of this?
It is possible that one testicle was damaged by your accident with a bicycle. However, the two testicles in a male are not always the same size. It would require your doctor to determine if one of your testicles was damaged or not.
Even if you lost one testicle, the other testicle has enough capacity to make up for the lost or damage testicle. It can produce sufficient testosterone for your body and sufficient sperm to get your wife pregnant. Most likely the miscarriage was not due to a potentially damaged testicle.
There is a spiritual issue, however, that we need to discuss. Why are you having sex with a woman you are not married to and trying to children in this unstable relationship? "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). If you want to raise a family with this woman, the proper response is to marry her.