Echocardiography measurements of the fetal heart for cardiac function in the left ventricle are not able by planimetric. Fetuses from diabetic mothers may have cardiac hypertrophy with microscopic changes that are similar to those found in adults with diabetic cardiomyopathy (i.e., myofibril disorganization). Our aim of this study is to describe reference ranges for cardiac output of the left ventricle by planimetric between 19 weeks and 37 weeks. 17 diabetic, 4 multiple gestations and 43 normal patients participated in the study. All studies will be performed using an ATL HDI 5000 ultrasound system. The technique is made by tracing the endothelial border of the left ventricle. The end of the diastole was assumed just after closure of the mitral valve, just prior to the onset of systole and end of the systole was the minimum cavity size visualized during the cardiac cycle. We choice the left ventricle because the tracing in the end cardio is better. The right ventricle has the moderator band doing most of the time very difficult to know where is the real cavity. All data were analyzed in computer statistical program (Epiinfo). The cardiac output at 19 to 23 weeks was 23.4 mL/min. (Low value was 9.8 mL/min. Maximum range was 59.3 mL/min. Standard deviation was 23.4 mL/min). Cardiac output at 24 to 32 was 111.3 mL/min. (Low value was 75.8 mL/min. Maximum range was 168.5 mL/min. Standard deviation was 81.9 mL/min). Cardiac output at 32 to 37 weeks was 656.8 mL/min. (Low value was 380.9 mL/min. Maximum range was 782.2 mL/min. Standard deviation was 299.9 mL/min). The median cardiac output value was 302.8 mL/min for 18 to 37 weeks. Cardiac output increases with the gestational age. Same happens with the standard deviation in each value calculated for each group. That means that measurements for cardiac output are more safe and precise when the echocardiogram is made in early stages of the pregnancy.