What Causes Recurrent Miscarriages?

Hi my wife has gone through 3 miscarriages in the last 4 yrs! And it always happens by the 2nd month when she has a spotting etc. She has been tested for thyroid, rubella, TB etc....but everything is fine. The last time even the foetus was sent for some genetical testing...that also turned out fine! I really don't knw what needs to be done!