Maternal age in trisomy.

By comparison with a more general theory, data on trisomy in live births, amniocenteses, and spontaneous abortions by year of maternal age are shown to fit a logistic augmented by a proportion independent of maternal age. The frequency of trisomy increases monotonically, with no discrepancy at extremely low or high maternal age. Trisomy 16 is exceptional in that all cases appear to be age-dependent. For groups A, B, and C most trisomies arise by a process independent of maternal age. A small proportion of these trisomies and about half of trisomies for smaller chromosomes (excluding trisomies 16 and perhaps 22) originate by some process dependent on maternal (but not paternal) age and therefore presumably independent of crossingover, which in the female takes place before birth.