Heterotrisomy recurrence risk: a practical maternal age-dependent approach for excess trisomy 21 risk calculation after a previous autosomal trisomy.

AbstractA new maternal age-dependent method to estimate absolute excess risks of trisomy 21, either after a previous trisomy 21 (homotrisomy) or after another trisomy (heterotrisomy), is proposed to be added to the estimated risk by conventional screening methods. Excess risk at term for a subsequent trisomy 21 was calculated from midtrimester risks reported by Morris et al., decreasing from 0.49% at 20 years to 0.01% at 46 years at the index pregnancy. Excess risk after a previous uncommon trisomy was derived from data reported by Warburton et al., decreasing from 0.37% at 20 years to 0.01% at 50 years.