Prenatal diagnosis in multiple pregnancy

Fetal abnormality is more common in multiple than in singleton pregnancies. This, together with the requirement to consider the risks with at least tw o babies to sample correctly each fetus and to undertake accurately-targete d selective termination, amounts to a major challenge for obstetricians inv olved in prenatal diagnosis. Early determination of chorionicity should be routine, since this influences not only the genetic risks but also the inva sive procedure chosen for karyotyping or genotyping. Assessment of nuchal t ranslucency identifies individual fetuses at risk of trisomy. Contrary to e xpectation, invasive procedures in twins appear to have procedure-related m iscarriage rates that are similar to those in singletons. Instead, contamin ation remains a concern at chorion ic villus sampling. Elective late karyot yping of fetuses may have a role in some countries. Whereas management opti ons for discordant fetal abnormality are relatively straightforward in dich orionic pregnancies, monochorionic pregnancies are at risk of co-twin seque lae after any single intrauterine death. Techniques have now been developed to occlude completely the cord vasculature by laser and/or ultrasound guid ed bipolar diathermy. Given the complexities associated with prenatal diagn osis, all invasive procedures in multiple pregnancies should be performed i n tertiary referral centres.