Introduction: The frequent occurrence of prenatal hypertrophy of the muscular ventricular septum has been widely reported in fetuses of diabetic mothers. Objectives: This experimental study was carried out to test the hypothesis that the weight of the heart, the ratio of the cardiac weight to that of the body, the thickness of the muscular ventricular septum, and the myocytic profile within the ventricular septum are all increased in fetuses of diabetic rats in comparison to fetuses of normal rats. Methods: Diabetes was induced in 5 pregnant Wistar rats, bearing 30 fetuses, on the eighth day after conception, by intraperitoneal injection of 50mg/kg of streptozotocin. Five normal pregnant Wistar rats, bearing 20 fetuses, made up the control group. Morphometric data were obtained by a computer-assisted method applied to the measurements of the thickness of the ventricular septum, and myocytic nuclear area. Statistical analysis utilized Student's t-test and Kruskal-Wallis test. Results: The mean thickness of the septum was 675.56 μm (± 159) in the control fetuses, and 904.39 μm (± 262) in the fetuses carried by diabetic mothers (p < 0.001). The cardiac weight was 0.016 g (± 0.004) in the control group, and 0.023 g (± 0.005) in the group of diabetic fetuses (p < 0.001). The ratio of cardiac to body weight was 0.294% (± 0.079) in the control group, and 0.514% (± 0.073) in the diabetic group (p < 0.001). The myocytic nuclear area was 14.70 μm2 in the control group, and 21.43 μm2 in the diabetic group (p < 0.001). Conclusions: The presence of cellular and morphologic cardiac hypertrophy in fetuses of diabetic rats was demonstrated by the significant difference between the two groups for each analyzed feature.