Why do you think that relatively few fetuses with chromosomal trisomies survive to birth?

This is  because  the  body  of  the  mother  is  able  to  detect  the  abnormalities of  the  fetus which  led   to  spontaneously  abort.The  few  that  survive  is  because  the  few  that survive  is  because the  body  of  the  mother  failure  to  do  what  is  programmed  to  do.