The umbilical pump: a contributor to twin-twin transfusion.

Background: Umbilical vascular coiling may function as a rudimentary pump that facilitates venous return from the placenta.
Cases: Three consecutive twin gestations with twin-twin transfusion syndrome were evaluated prospectively at delivery. The birth weights and umbilical coiling indices of donor and recipient twins were compared. The umbilical coiling index was determined by dividing the number of complete vascular coils in a given umbilical cord by the cord's length in centimeters. In each case, the recipient twin was larger at birth and had an umbilical coiling index value that was at least twice that of the corresponding donor twin.
Conclusion: Differential umbilical vascular coiling densities among monochorionic twins may play a role in the pathogenesis of twin-twin transfusion syndrome.