Why are som genetic or chromosomal mutations more common than the others, e.g the trisomy of 21 or 13, 18 are more usual than trisomy of 1 (in fact i’ve never heard about it:). May the reason be, that some mutations are so highly fatal, that they prevent any development of embryo (even the earliest stages). And when the embryo is not developed at all (we even don’t notice that the woman is pregnant, nay that the embryo has some genetic disorder) we think that those mutations do not occure (even if they occurance may be the same as of those mutations which enable baby to develop, be born and survive for some time (or longer))