What Is The Cause Of Recurrent Miscarriages?

I have had 3 consecutive miscarriages in the last 2yrs, I had a healthy baby boy in 08` normal pregnancy and no problems during the pregnancy or after, my son is still normal and healthy as far as we know to this day. However, when my partner and I tried to have another baby I was diagnosed with a blighted ovum. The doctor told me it was no big deal this happens a lot, well it happened twice more, same diagnosis blighted ovum, they were very confused on why my HCg levels continued to double every 48hrs as they should. I had 2 D&C`s and this last time i decided to let my body miscarry naturally. I am getting chromosome testing done to see if this is the cause of the recurrent miscarriages I am just wondering how my son is normal and I had him with no problems if I have a problem with chromosome count? And also, when I got pregnant with my son the egg had fallen from the left ovary, all 3 of my miscarriages were eggs from the right ovary and I`m not sure if that could even be a factor, I`m just very worried I`m 21yrs old and the thought of not being able to expand my family scares me daily. I am having a hard time waiting to get my testing done because its disheartening I do understand why I have to wait. However If you can please answer any of this I would truly be thankful.