What were the causes of your miscarriages?

I have had 2 miscarriages, one at 4 weeks and the other at about 5 weeks. I am now pregnant for the 3rd time and I am 13 weeks and 3 days pregnant. What are my chances of have a healthy and full term pregnancy. And I know this is a hard subject but for those of you that has had a miscarriage do you know what the cause was? and how far along were you? I am just so scared. I try to be happy and only think of happy thoughts. My Doctor says everything look great, my blood work looks great and my l test looks great. Any uplifting thoughts would be greatly appreciated. Thanks so much!