Twins, placentas, and genetics: acardiac twinning in a dichorionic, diamniotic, monozygotic twin gestation.

We describe a human acardiac twin with associated vascular anastomoses in a dichorionic diamniotic fused twin placenta. A 22-year-old woman delivered a healthy 3,554 g male infant and a fused diamniotic dichorionic twin placenta with a 230 g umbilical cord-attached, skin-covered, ovoid mass, consistent with acardiac amorphus. By gross and histological examination, the placental dividing membranes comprised four leaves, one amnion from each placenta, and two centrally fused chorions, diagnostic of dichorionicity. Placental barium injection of the normal twin's umbilical vein showed an anastomosis with the acardiac twin which traversed the dividing membranes, then supplied major vessels of the acardiac mass via its 5.5 cm umbilical cord. DNA-typing studies of the normal twin's placenta and of the acardiac twin's tissues revealed identical alleles at 11 distinct genetic polymorphic loci, consistent with monozygosity. Our findings demonstrate that vascular anastomoses can occur in dichorionic twin placentas, and that human acardiac twinning is not, as heretofore believed, restricted to monochorionic placentas.