This is a much too complicated and potentially dangerous topic for an a
adequate explaination over the NET. A place to begin would be in the
chapters in Chromosome Abnormalities and Genetic Counselling by Gardner
R.J.M. and Sutherland, G.M.ISBN 0195049321 published by Oxford
University Press and available through most Med School Libraries. If
you tell me more about your city I can put you in touch with the local
genetic counselling clinic.
THe fact that your friends were ascertained because of recurrent
miscarriage is one big factor in their favor.