Heterokaryotypic Monochorionic Twin Pregnancy: New Perspective

Monozygotic twins are thought to be identical since they are created from a single fertilized egg, yet there may be differences in their congenital defects, birth weight, and genetic makeup. Asymmetric X chromosome inactivation, unequal gene imprinting, and postzygotic mitotic mistakes including nondisjunction and anaphase lag can all result in heterokaryotypic monochorionic twins. We report a monochorionic twin pregnancy that exhibited stigmata associated with trisomy 18 on postnatal examination despite a low risk of common aneuploidy (trisomy 18) on noninvasive prenatal screening. Short tandem repeat markers were used for postnatal examination to confirm high-grade mosaicism. These markers indicated mosaic trisomy 18 in twin II and normal in twin I, ruling out uniparental disomy and establishing monozygosity in both fetuses. Twin sac amniocentesis is a prenatal diagnostic procedure that can be used to identify discrepant monochorionic twins because chorionic villus sampling, single sac amniocentesis, or cordocentesis may not be able to rule out aneuploidy in the second fetus and may yield a false-negative result. For prompt zygosity diagnosis, chromosomal complement, genetic counseling, and referral for selective fetal reduction, twin sac amniocentesis is recommended.