Assessment of cardiac dimensions of the chambers, great arteries and veins in the human fetus is important to distinguish abnormal dimensions from normal. This study establishes normal values based on cross-sectional echocardiographic measurements over the gestational period where these measurements may be clinically useful. Ventricular and atrial dimensions were measured from the 4-chamber view, the short-axis dimension immediately below the mitral and tricuspid valve leaflets in diastole, and the long axis from the closed apposed atrioventricular valves to their respective apices. The ventricular walls and septum were measured at the level at which cavity dimensions in diastole were measured, defining both the left and right ventricular wall thickness, as well as that of the ventricular septum. Furthermore, the long axis of the right and left atria was measured from the center of the apposed atrioventricular valve leaflets to the posterior atrial wall, and the sizes of the atrial chambers were defined using their widths at the prospective broadest points through the area of foramen ovale. From a variety of views, diameters were measured at maximal expansion of the main, left and right pulmonary arteries, the ductus arteriosus, and the superior and inferior venae cavae. The data were evaluated longitudinally from 18 weeks to term, and regression analysis was performed using the best fit of a linear or polynomial equation. The data provide a means for evaluating the normal sizes and dimensions of the fetal heart chambers, as well as the thickness of the ventricular walls and septum.