Some aspects of twin-gestation in the mare.

A study has been made of 44 cases of twin pregnancy in mares. The fetuses, foals, fetal membranes and blood groups of parents and foals were examined in order to determine the incidence of chorio-vascular anastomosis, blood chimaerism and freemartinism. All of the twins proved to be dizygotic and 33 of the 34 pregnancies were bicornual. Fusion of the chorion was seen in 14 of 22 placentas and macroscopic choriovascular bridges were observed in five of these, although blood chimaerism was diagnosed in 11 of 25 cases (44 per cent) examined. This would indicate a high incidence of microchoriovascular anastomosis. The fetal female genital organs of four heterosexual twins were found to be anatomically and histologically normal at autopsy despite the existence of blood chimaerism. Five other sexvalley mature mares born as heterosexual twins with chimaerism all had normal genital organs and estrus cycles and one of these has since become pregnant. The important interspecies differences with respect to the incidence and degree of choriovascular anastomosis and freemartinism are stressed and suggestions for a plausible explanation were advanced.