The vanishing twin: morphologic and cytogenetic evaluation of an ultrasonographic phenomenon.

Twin pregnancy was observed by ultrasonographic examination in the 6th week of gestation. After singleton term delivery a thickening of the membranes opposite to the main placenta showed degenerated chorionic villi embedded between one layer of amnion and chorion; no fetal parts were observed. Villus cells from both placentas were mainly diploid; 2 of 30 were tetraploid. However, 19 of 30 cells from membranes overlying the satellite placenta were tetraploid. Marker analysis was consistent with duplication of a normal conception diploid chromosome complement as the mechanism for tetraploidy. Postconceptional nondisjunction leading to tetraploidy in one twin conceptus may explain demise in early pregnancy. Tetraploidy observed by chorionic villus biopsy must be confirmed by amniocentesis before interruption of the pregnancy is considered.