Why use it

Replaces the outdated "add 1%" rule with precise, age-adjusted absolute excess risk counseling for prenatal testing decisions. Uses actual published data tables rather than approximations.

About

Estimates the risk for a pregnant woman who had a previous pregnancy affected by an aneuploidy to have another affected pregnancy. Uses complete age-stratified lookup tables from Morris et al. (2005b) Table 13-10 for homotrisomy T21, Table 13-9 multipliers for T13/T18, and Grande et al. (2017) Table 13-11 for heterotrisomy after non-T21 trisomies. The risk is NOT a fixed 1%; it depends vitally on maternal age at the index pregnancy and current maternal age.

When to use

Preconception or prenatal counseling for women with a history of a trisomic pregnancy.
Deciding whether to offer NIPT, CVS, or amniocentesis based on individualized risk.
Counseling after miscarriage with documented trisomy on products of conception.

Formula

For Previous T21 (Homotrisomy): Risk = Table13-10_ColA(current_age) + Table13-10_ColB(index_age) For Previous T13/T18/Other (Heterotrisomy to T21): Risk = Table13-3(current_age) + Table13-11(index_age) For Previous T13/T18 (Homotrisomy): Risk = Table13-3(current_age) × Fold_Multiplier(Table13-9)

Pearls & pitfalls

If the previous T21 occurred at a young maternal age (<30), the biological excess risk is highest (0.62%), suggesting gonadal mosaicism or genetic predisposition to nondisjunction.
If the previous T21 occurred at advanced maternal age (>35), the excess risk drops dramatically and is mostly driven by the current age-related baseline.
For heterotrisomy (T21 after prior T13/T18/other), Grande et al. (2017) showed that excess risk also declines with advancing maternal age at the index pregnancy.
The combined risk = Column A (baseline age-specific) + Column B (excess from prior event). Example: 30yo woman who had DS at 25 → 0.14% + 0.57% = 0.71% (1 in 141).
Gonadal mosaicism is presumed to be an uncommon cause but may explain extreme recurrences (e.g., 3+ trisomic conceptions in young mothers).
Recurrence of T13 or T18 is very rare (T13: 8.6-9.5× baseline, T18: 1.7-3.1× baseline), but the absolute risk remains small due to low baseline rates.

Next steps

Offer NIPT or invasive testing (CVS/amniocentesis) based on the calculated risk and patient preference. If risk exceeds 1/250, invasive testing is typically offered.

Evidence

Morris JK et al. (2005b) Table 13-10: complete age-specific excess risk data for T21 homotrisomy. Grande et al. (2017) Table 13-11: age-specific excess risk for heterotrisomy. Warburton et al. (2004) provided the foundational cohort data. De Souza et al. (2009) contributed additional validation.

References

Morris JK, Mutton DE, Alberman E. Recurrences of free trisomy 21: analysis of data from the National Down Syndrome Cytogenetic Register. Prenat Diagn. 2005;25:1120-1128.
Grande M, Stergiotou I, Borrell A. Risk of heterotrisomy after previous aneuploid pregnancy. J Matern Fetal Neonatal Med. 2017.
Warburton D, Dallaire L, Thangavelu M, et al. Trisomy recurrence: a reconsideration based on North American data. Am J Hum Genet. 2004;75:376-385.
Morris JK, Savva GM. The risk of fetal loss following a prenatal diagnosis of trisomy 13 or trisomy 18. Am J Med Genet A. 2008;146A:827-832.

Post-Trisomy Recurrence Risk (Morris / Grande)

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