A total of 367 human fetuses and newborn subjects weighing from 60 to 5000 g provided the material for a morphometric study of the heart. A total of 17 interventricular parameters were measured in each specimen, one of the parameters representing an innovation with regard to the classically used set of measurements. A new anatomo-geometric configuration is described for each ventricle along with a new component for the left ventricular outflow tract, designated as the aortic outflow tract. The appropriate stereometric formulas were used to calculate real ventricular volumes rather than the previously studied volumetric indices. Additionally, correlation indices were calculated for ventricular wall thickness as well as the circumferences and diameters of the atrioventricular and arterial valves. The results show that, in fetuses of up to 2700 g in body weight, ventricular wall thickness is greater in the right than in the left ventricle, although the opposite is true in fetuses weighing above 2700 g. Throughout the range of weights studied, ventricular volume was greater in the left than in the right chamber. Tricuspid and pulmonary valve circumference and diameter were consistently greater than in the mitral and aortic valves, respectively. We believe the new morphometric data and their innovative interpretation to have immediate applications in both the morphological and functional areas of cardiology.