What causes multiple miscarriages after a healthy birth?

My friend just suffered her second miscarriage. She had a healthy baby boy 4 years ago. About 2 years later, she had another baby, but he died a month later of a rare heart defect. Since then, she's had 2 miscarriages. I was just wondering what would cause this. She does all the right things...goes to the doctor, takes care of herself, avoids stress, etc. The doctors haven't really told her anything.

If it matters, she's 27.

The majority of miscarriages is spontaneous abortion to which her body is aware that there is a genetical defect to the fetus and whcih case it desides to remove it from the body. What she and her husband need to do is go to a genetic counslor and they will run genetical tests. The fact that she had a child that died of a rare heart disease is a HUGE signal. Probably the doctors aren't thinking logially about his situation and not really seeing the overall picture.

Since she's had two miscarriages and a child with an unusual birth defect, I'd suspect that either she or her partner might be carriers for some genetic flaw. The vast majority of miscarriages in the first trimester (three months) are because the fetus had some genetic issue incompatible with life. The fact that they've had a healthy baby doesn't rule that out.

Later miscarriages are often a different issue, and it's not impossible that she's just having a horrible run of bad luck. Only about 1 in two fertilized eggs goes on to produce a healthy baby. But miscarrying is not due to stress... it's a medical issue. I'd suggest she talk to her doctor about whether the miscarriages might not have a genetic component, and go from there.