Monozygotic twins discordant for trisomy 21: Discussion of etiological events involved

Objective To elucidate the etiologies of discordant trisomy 21 in monozygotic twin pregnancy. Case report A monochorionic diamniotic twin pregnancy with hydrops and cleft lip (twin 1) found in one fetus presented at gestational age of 17 weeks. Amniotic fluid karyotyping showed nonmosaic trisomy 21 in twin 1 (47, XY, +21 [20]) and a normal karyotype in twin 2 (46, XY [20]). Short tandem repeat (STR) polymorphism markers revealed that the two fetuses were monozygotic, and the two chromosomes 21 were maternal isodisomy in the trisomy fetus. The chromosomal constitution of placentas in the territory of trisomy 21 cotwin was 47, XY, +21 [20] and was a mosaic 47, XY+21 [12]/46, XY [8] in the normal karyotyped twin. Conclusion Our case of monozygotic twin with discordant trisomy 21 might start with a prezygotic maternal meiosis II non-disjunction error-caused trisomy 21 zygote, and after twinning, one remained trisomy 21, and the other twin underwent trisomy rescue and became a mosaic trisomy 21 in morula or early blastocyst stage before the formation of pre-embryo, which subsequently resulted in mosaic trisomy 21 of the placental tissue and normal chromosomal constitution of the fetus.