ABSTRACT
In general, twins have a two-to three-fold higher rate of congenital malformations than that of singletons, mainly because of the higher incidence (by about 50%) of malformations in MZ compared with DZ twins6,7. The reasons for the higher morbidity and mortality in twins compared with singletons and in MZ compared with DZ twins remain unclear. However, the three leading theories are as follows8,9:
(1) Vascular theory: the hemodynamic imbalance caused by vascular anastomoses impairs normal organogenesis.
