Nuchal translucency thickness and crown-rump length in twin pregnancies with chromosomally abnormal fetuses.

We retrospectively examined the crown-rump length and nuchal translucency thickness of each fetus in eight twin pregnancies in which karyotyping at 10 to 14 weeks' gestation demonstrated that at least one of the fetuses was chromosomally abnormal. Eight fetuses had trisomy 21 and two had trisomy 18. The nuchal translucency thickness was more than 2.5 mm in nine (90%) of the trisomic fetuses and in one of the chromosomally normal ones. In contrast, the crown-rump length was below the fifth percentile in only one of the fetuses with trisomy 18; all other measurements were within the normal range.