Invasive procedures in twins are complex and should only be performed in fetal medicine centres. Great care is taken to avoid mislabelling and misidentification of an affected twin. While this may be facilitated in the presence of discordant gender or structural abnormality, it is good practice always to map the topography in terms of location within the uterus, placental site, cord insertion and plane of the dividing septum. This is a prerequisite for interpretation of discordant results and for selective feticide. Ideally the operator doing the diagnostic procedure should also undertake any selective feticide to minimize uncertainty and obviate any need for confirmatory inva-sivetesting. In structurally concordant MC twins, only one needs sampling for prenatal diagnosis, but the operator should be certain of this on first trimester scanning. With this exception, it is important to ensure that both fetuses are sampled separately. With amniocentesis this is best achieved by two separate ultrasound-guided procedures as far away as possiblefrom thedividing septum, although there are series using single-entry techniques with low rates of complications. With fetal blood sampling, the intrahepatic vein can be sampled to avoid confusing the cord origins. Most operators consider chronic villus sample (CVS) contraindicated in DC twins, because of a 1-5% rate of contamination [20,21], and thus potential for false positive and negative results through inadvertently sampling the same twin twice. Otherwise DNA fingerprinting and/or confirmatory amniocentesis may be necessary in DC twins with concordant-sex karyotypes at CVS.
Miscarriage rates after amniocentesis in twins seem higher than in singletons , although the lack of randomized trials or large cohorts preclude estimation of the procedure-related risk.
Selective feticide in DC twins discordant for fetal abnormality by injection of intracardiac KCL is associated with a 7% loss rate in the international registry, with marginally lower rates if the procedure is done before than after 13 weeks . The same technique in MC twins leads to death of the healthy twin due to agonal exsanguination along vascular anastomoses. To obviate this, a variety of cord occlusion techniques have recently been developed to render selective termination in MC twins now feasible.
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