Cohen & Daniel Viability Thresholds

Why use it

Essential for stratifying the risk of an unbalanced pregnancy resulting in a live birth with multiple malformations versus a recurrent pregnancy loss, guiding reproductive counseling for translocation carriers.

About

This calculator evaluates the fetal survival potential of chromosomal imbalances resulting from reciprocal translocations. It is based on the "viability triangle" concept, where maximum thresholds of autosomal monosomy and trisomy determine whether the pregnancy is likely to result in a malformed live birth or in utero lethality (spontaneous abortion). The human fetus tolerates less loss of genetic material (monosomy) than gain (trisomy), establishing asymmetric thresholds.

When to use

• Reproductive counseling for couples carrying reciprocal translocations.
• Estimating the risk of a malformed offspring versus the risk of recurrent miscarriage.
• Evaluating adjacent-1 or adjacent-2 segregation products from meiotic quadrivalent analysis.
• Providing quantitative context when discussing PGT-SR (Preimplantation Genetic Testing for Structural Rearrangements).

Formula

Pearls & pitfalls

• This model assumes genomic tolerance is strictly quantitative (the size of the imbalance matters more than the exact gene content for overall viability, though gene-rich chromosomes are exceptions).
• Imbalances exceeding these thresholds tend to cause early embryonic or fetal demise.
• Use in conjunction with the Haploid Autosomal Length (HAL) Calculator to obtain exact monosomy and trisomy percentages.
• The thresholds are NOT absolute: some chromosomes rich in dosage-sensitive genes (e.g., 17, 19) may be lethal at lower imbalance percentages.
• Outlying points exist in the literature where both segments are substantial yet survivable (e.g., t(5;10)(p13;q23.3) with 1.1% monosomy + 1.4% trisomy = 2.5% HAL total).
• Duplications (trisomy) are generally more tolerated than deletions (monosomy) because haploinsufficiency tends to be more deleterious than triplosensitivity.

Next steps

If classified as viable, the couple should receive detailed genetic counseling regarding severe morbidities and congenital malformations. If lethal in utero, explain the high risk of recurrent pregnancy loss and discuss reproductive alternatives like PGT-SR.

Evidence

Based on consolidated empirical survival data of chromosomal imbalances from multiple international series. The viability triangle has been validated across hundreds of translocation families (Stengel-Rutkowski et al. 1988, Cohen et al. 1992, 1994, Midro et al. 2000).

References

1. Gardner RJM, Sutherland GR, Shaffer LG. Chromosome Abnormalities and Genetic Counseling. Oxford University Press.
2. Cohen O, Cans C, Cuillel M, et al. Cartographic study: breakpoints in 1574 families carrying human reciprocal translocations. Hum Genet. 1996;97:659-667.
3. Stengel-Rutkowski S, Stene J, Gallano P. Risk estimates in balanced familial reciprocal translocations. Expansion Scientifique Francaise, Paris. 1988.