ABSTRACTThp/+mouse is known to have differing phenotypes depending on gender of the ThPparent. In the present study, fetuses with maternally [Group A (until day 18 of pregnancy) and Group C (day 19)] and paternally [Group B]inherited ThPwere examined with particular reference to the developmental abnormalities of hearts. In addition, a small number of fetuses with maternally inherited twLub2on day 19 were compared with Thp. Group A ThPfetuses had greater body weight, possibly larger body size, generalized edema (100%), marked enlargement of the hearts (100%) and hypoplastic pulmonary trunk (73.7%). There were no such malformations in Group B ThPfetuses or in the controls (+/+) for both groups. The bilaterally thickened ventricular wall of fetal hearts in Group A and C ThPfetuses bulged into ventricular cavity. The pulmonary valve was also thickened. The labeling indices of the ventricular myocardial cells by BrdU were inclined to be higher in Group A ThPthan in the control (+/+) fetuses. Fetuses of twLub2had abnormalities of the cardiovascular system similar to Group C Thp. The results suggest that cardiac lesions in fetuses with maternally inherited ThPand twLub2correspond to cardiomyopathy. Overgrowth and enlarged heart indicate the possibility of ThPas an animal model for Wiedemann‐Beckwith syndrom