A Duplex Color apparatus equipped with real-time imaging and Doppler sector scanner was used to scan fetal hearts, ranging from 17 to 41 weeks gestational age. A total of 323 normal fetuses were studied. The four-chamber view was obtained in a horizontal section just above the level of the fetal diaphragm. Five variables of the Chinese fetal heart in relation to the width of the right ventricle, width of the left ventricle, ratio of right ventricle/left ventricle (RV/LV), length of the fetal heart and the cardiac volume of a four-chamber view were set against gestational age in weeks and expressed in regression equations. The ratio of RV/LV is quite constant in relation to the gestational age. The mean ranges between 0.9916 for 17 weeks gestation and 1.0045 for a term fetus. In 10 abnormal cases with congenital cardiac defects, using the 5th and the 95th percentiles of this normal data as cutoff points, the RV/LV ratio had the highest sensitivity rate of 70% (7/10) in predicting fetal cardiac anomaly. The width of the left ventricle was the second most sensitive parameter with a sensitivity of 4/10 (40%). The RV/LV ratio of a four-chamber view is a simple, time-saving screening parameter for predicting congenital cardiac defects antenatally.