Maternal Age and Trisomy The association between increasing maternal age and trisomy is the most important etiologic factor in congenital chromosomal disorders. Among women under the age of 25, ~2% of all clinically recognized pregnancies are trisomic; by the age of 36, however, this figure increases to 10% and by the age of 42, to 33% (Fig. 63-5). This association between maternal age and trisomy is exerted without respect to race, geography, or socioeconomic factors and likely affects segregation of all chromosomes. Figure 63-5 Estimated maternal age–adjusted rates of trisomy among all clinically recognized pregnancies (., spontaneous abortions, stillbirths, and livebirths). . | Chapter 063. Chromosome Disorders Part 8 Maternal Age and Trisomy The association between increasing maternal age and trisomy is the most important etiologic factor in congenital chromosomal disorders. Among women under the age of 25 2 of all clinically recognized pregnancies are trisomic by the age of 36 however this figure increases to 10 and by the age of 42 to 33 Fig. 63-5 . This association between maternal age and trisomy is exerted without respect to race geography or socioeconomic factors and likely affects segregation of all chromosomes. Figure 63-5 Estimated maternal age-adjusted rates of trisomy among all clinically recognized pregnancies . spontaneous abortions stillbirths and livebirths . Among women in their forties over 25 of all pregnancies are estimated to involve a trisomic conception the vast majority of these spontaneously abort with only trisomies 13 18 and 21 and sex chromosome trisomies surviving to term with any appreciable frequency. Despite the importance of increasing age little is known about the mechanism by which aging leads to abnormal chromosomal segregation. As noted above it is thought to originate in maternal meiosis I owing to the protracted time to completion often 40 years in females and recent studies suggest that it may be associated with alterations in meiotic crossing-over. In trisomy 21 for example crossover patterns appear to be similarly abnormal in younger and older mothers of trisomic conceptions. Thus it has been suggested that two distinct steps or hits may be involved in maternal age-related nondisjunction. The first hit which is age independent involves the establishment of a vulnerable crossover configuration in the fetal oocyte the second hit which is age dependent involves abnormal processing of the vulnerable bivalent structure at metaphase I. If this model is correct it suggests that the nondisjunctional process is the same in younger and older women but it occurs more frequently with aging possibly because
