Ultrasonographic assessment of the ipsilon zone is useful for predicting chorionicity in triplet pregnancies.

Objective To examine the usefulness of selecting the appropriate techque for fetal karyotyping in twin pregnancies by using maternal age and fetal nuchal translucency thickness to determine risk for chromosomal defects in each fetus. Setting Fetal Medicine Centre, London, United Kingdom. Subjects Sixty‐seven twin pregnancies identified at the time of an ultrasound scan for determination of fetal nuchal translucency thickness, where the parents requested karyotyping. Intervention The risk for chromosomal defects in each fetus was calculated from the maternal age and fetal nuchal translucency thickness at 10 to 14 weeks of gestation. If the estimated risk for either fetus was 1 in 50 or greater, chorion villus sampling was the method of choice, whereas if the risk was less than 1 in 50 second trimester amniocentesis was performed. Results The estimated risk for trisomies was more than 1 in 50 in 34 pregnancies and 23.5% of these fetuses were found to be chromosomally abnormal. In contrast, in the 33 low risk pregnancies chromosomal abnormalities were found in only 1.5% of the fetuses. Conclusions In twin pregnancies the technique for fetal karyotyping may be selected by calculating the risk for chromosomal abnormality based on maternal age and fetal nuchal translucency thickness.

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