Recent cytogenetic studies on spontaneous and induced abortuses indicate by implication that almost half of the fertilized ova are chromosomally abnormal and that 90% of the abnormals are eliminated before the pregnancy is recognized. Around 5% of conceptuses at the beginning of clinically recognizable pregnancy are chromosomally abnormal. Of these 5%, half are autosomal trisomies involving chromosomes 2 through 22. Trisomy 1 has never been encountered. Among the trisomies, trisomy 16 is most frequent accounting for 30% of all trisomies, trisomies 21 and 22 are next common, being 10% each, and trisomies 13 and 18 each take 5%. Monosomy X is found 19% of all abnormals, triploidy is 17% and tetraploidy 5%. More than 90% of such abnormal conceptuses later abort spontaneously, most of them during the first trimester of pregnancy. Thus, only 0.6% of liveborns are chromosomally abnormal. Prenatal selection rates of these chromosome abnormalities are: trisomy 21 : 77%, trisomy 18 : 94%, trisomy 13 : 96%, monosomy X : 99% and triploidy : 99.8%. Most of the abortuses with lethal chromosome abnormalities are either an empty sac or a totally disorganized embryo. They stop to develop at 6 weeks of embryonic age or earlier. On the other hand, apparently normal fetuses are occasionally found among conceptuses with trisomy 21, monosomy X and tri ploidy. In conclusion, spontaneous abortions are an efficient device for the elimination of abnormal conceptuses.
