What Are The Reasons For Recurrent Miscarriage?

I have a friend who is diabetic and has a thin skinned uterus. She told me that she continued to miscarry babys because of the thin skin due to diabetes. My family has a history of diabetes and on my last exam my doctor said she could see that I had it in my family just by looking at my skin.I don't have diabetes. But, I continue to miscarry my own baby's.