Fetal cardiac output was quantitated by 2-dimensional (2-D) Doppler echocardiography. Circulatory dynamics in the fetus differ from circulatory dynamics after birth because right and left ventricles work in parallel. Therefore, volume flow was estimated over the mitral and tricuspid orifice separately. Serial 2-D echo Doppler studies were performed in 28 normal fetuses at 4-week intervals from 15 to 18 weeks of gestation to the time of parturition. Measurement of blood flow velocities through mitral and tricuspid valve orifices and measurement of the diameter of these valve orifices were obtained from apical 4-chamber views of the fetal heart. Angle-corrected mean temporal blood flow velocities increased linearly with gestational age, whereas the area of blood flow and calculated right and left ventricular output increased exponentially. The right ventricular blood flow velocities, areas of tricuspid orifice and calculated right ventricular outputs were significantly higher than those for the left ventricle when analyzed by paired t test (0.005 greater than 2p greater than 0.001). The ratio of right to left ventricular output decreased from 1.34 +/- 0.28 (+/- standard deviation) at 15 weeks to 1.08 +/- 0.28 at 40 weeks. The combined ventricular output of the fetus near term is approximately 1,735 ml/min, whereas flow indicated for estimated fetal weight is constant at a mean value of 553 +/- 153 ml/min/kg. Thus, 2-D echo Doppler provided a means of estimating fetal cardiac blood flow at the atrioventricular orifices, and blood flow measured at these orifices increased exponentially during gestation, with a larger output from the right ventricle than from the left ventricle.