To describe a method for calculating fetus-specific Down syndrome risk in triplets, allowing for nuchal translucency (NT) correlation between fetuses.Methods
The between-fetus correlation coefficient of log NT, in multiples of the median, was estimated from a series of 95 unaffected triplets. A trivariate log Gaussian model was used to calculate likelihood ratios for discordant and concordant Down syndrome. Applying these to the prior maternal age-specific risk yielded risks in monozygous, dizygous and trizygous triplets. The weighted average risk was then computed with weights relating to chorionicity and assisted reproduction. The method was illustrated.Results
The correlation coefficient in unaffected pregnancies was 0.408 (P < 0.0001; 95% confidence interval 0.30–0.50) and estimated to be 0.205 and 0.107 in triplets pairs with one or two affected fetuses. The illustration showed that very large differences could be obtained in the risks when the extent of correlation in NT between fetuses is taken into account compared with when the measurements are treated as independent.Conclusion
Fetus-specific Down syndrome risks in triplets should be calculated using its own NT value and that in the other fetuses, chorionicity, and method of conception. © 2012 John Wiley & Sons, Ltd.