Diamniotic conjoined fetuses in a triplet pregnancy: an insight into embryonic topology.

We present a case of triplets, 2 of whom were monochorionic diamniotic conjoined fetuses, and the other triplet was in a separate chorion. The pregnancy followed in vitro fertilization with 2 embryo transfers and the conjoined fetuses developed from a single embryo. An early ultrasound showed 2 embryos attached to 1 yolk sac. Further monitoring of the pregnancy showed these 2 triplets to be diamniotic with a short umbilical cord/body stalk anomaly. After fetal loss a postmortem examination demonstrated that these fetuses were conjoined with a body stalk anomaly/short umbilical cord syndrome with fusion of the coelomic cavities and small bowel (minimally united/conjoined twins). There were dividing amniotic membranes. This case demonstrates the development of an unusual type of conjoined twin with characteristic features. Conjoined fetuses with this pattern have fused small bowel and cloacal anomalies and often diamniotic placentation, when this is recorded. Various terms including minimally united/conjoined omphalopagus/ischiopagus have been used for this characteristic pattern, but this pattern may warrant a specific term. "Diamniotic vitellopagus" may best reflect the pathogenesis of this pattern.