Maternal age and trisomy

This shows maternal-age-specific estimates of trisomy among all clinically recognized human pregnancies, generated by combining data from individual trisomies and assuming a spontaneous abortion rate of ~15%. Not all individual trisomies manifest the same slope as seen here; for example, for trisomy 16, the commonest of all human trisomies, the increase is essentially linear. So, non-disjunctional mechanisms associated with maternal age must vary among different human chromosomes. There is also an apparent "bump" in trisomy among teenage girls. This slight increase has also been observed in several studies of Down syndrome, and might reflect a tendency to non-disjoin in the earliest ovarian cycles of the human female.