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 two babies to sample correctly each fetus and to undertake accurately-targeted selective termination, amounts to a major challenge for obstetricians involved in prenatal diagnosis. Early determination of chorionicity should be routine, since this influences not only the genetic risks but also the invasive procedure chosen for karyotyping or genotyping. Assessment of nuchal translucency identifies individual fetuses at risk of trisomy. Contrary to expectation, invasive procedures in twins appear to have procedure-related miscarriage rates that are similar to those in singletons. Instead, contamination remains a concern at chorionic villus sampling. Elective late karyotyping of fetuses may have a role in some countries. Whereas management options for discordant fetal abnormality are relatively straightforward in dichorionic pregnancies, monochorionic pregnancies are at risk of co-twin sequelae after any single intrauterine death. Techniques have now been developed to occlude completely the cord vasculature by laser and/or ultrasound guided bipolar diathermy. Given the complexities associated with prenatal diagnosis, all invasive procedures in multiple pregnancies should be performed in tertiary referral centres.