Down syndrome risk calculation for a twin fetus taking account of the nuchal translucency in the co-twin

Objective The objective of this study was to describe and illustrate a method for calculating fetus-specific Down syndrome risk in twins, allowing for between-fetus nuchal translucency (NT) correlation. Methods The between-fetus correlation coefficient of log NT, in multiples of the median, was estimated from a series of 325 unaffected twins after adjustment for sonographer bias. A bivariate 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 and dizygous twins. The weighted average risk was then computed with weights relating to chorionicity, gender, assisted reproduction and ethnicity. The method was illustrated using examples. Results The correlation coefficient in unaffected pregnancies was 0.45 (P < 0.0001) and estimated to be 0.12 and 0.04 in discordant and concordant twins, respectively. The examples showed very large differences in the risks obtained when the extent of correlation in NT between fetuses is taken into account and when the measurements are treated as independent. Conclusion Fetus-specific Down syndrome risks in twins should be calculated using its own NT value as well as that of the co-twin. Copyright (C) 2010 John Wiley & Sons, Ltd.