Help Trying to Conceive

My husband & I have been trying to get pregnant for a few years now with no luck until last July when we were blessed with twins. We were so excited & everything was great we did everything & anything to better ours & prepare for the arrival of our babies. But when I was at 13 weeks I miscarried without any warrings. My doctor couldn't even explain what happened especailly since I had just had a great check up 12 hours previously to the miscarriage.