"We hypothesize that the increased risk of spontaneous abortion with male age is a consequence of an increasing frequency of chromosomal anomalies in the spermatozoa with male age, which in turn increases the risk of spontaneous abortion. The influence of male age on the proportion of spermatozoa carrying a chromosomal anomaly or with damaged DNA has been documented in several groups of men. It may well result, in part, from the continuous replication of stem cell chromosomes from puberty onward; indeed, male stem cells have undergone about 150 chromosomal replications at the age of 20 years and about 600 by the age of 40 years. The chromosomal anomalies found in spontaneous abortions include autosomal trisomies, present in about 50 percent of abortuses with chromosomal anomalies, monosomy X (present in about 20 percent), and triploidy (16 percent). The remaining 12 percent correspond mainly to tetraploidy and structural anomalies of the chromosomes. The two most frequent anomalies sometimes have a paternal origin: Autosomal trisomies stem from nondisjunction during spermatogenesis in 10–20 percent or more of cases and, in monosomy X, the missing chromosome is most often the paternal one. Moreover, sperm chromosome aneuploidy may play a role in the etiology of recurrent pregnancy loss."