What Causes Frequent Miscarriages After Treated Dysplasia?

I had two perfectly healthy babies now 4 and 2 1/2. Since then I've had a stillbirth (twins) at 16 1/2 weeks pregnant and 2 (back to back) miscarriages. I just found out I'm pregnant again, and already started cramping and bleeding a lot. I'm only 22 years old, I do drink caffine, I smoke cigarettes ( but stop when I know I'm pregnant ) I also hae had dysplasia in the past that has cleared up. What could be causing my misfortunes...