OBJECTIVE--To measure lung blood flow and flow through the foramen ovale in the normal human fetus and to assess the changes in each with gestational age and the proportions of combined ventricular output that the respective flows represent. PATIENTS AND DESIGN--38 normal fetuses (gestational age 18-37 weeks) were studied prospectively with Doppler echocardiography. METHODS--Echocardiographic images and Doppler velocity signals were obtained from the ascending aorta, main pulmonary artery, and ductus arteriosus from each fetus and digitised to obtain arterial diameters, heart rates, and velocity-time integrals. Blood flow in each artery was calculated as the product of heart rate, flow-velocity integral, and arterial cross sectional area. Blood flow through the lung was assessed as the difference between flow in the pulmonary artery and ductal flow; combined ventricular output as the sum of aortic and pulmonary artery flows; and flow through the foramen ovale as the difference between flows through the aorta and lungs. RESULTS--Blood flow through the lungs increased exponentially with gestational age (r = 0.89, p < 0.001), by almost four-fold over the period of gestation studied, and was a mean (SD) of 22% (7%) of combined ventricular output. Blood flow through the foramen ovale increased exponentially by threefold (r = 0.77, p < 0.001), representing between 17% and 31% of combined ventricular output. CONCLUSIONS--Blood flow through the lungs and across the foramen ovale can be calculated non-invasively in the normal human fetus. Both flows increase exponentially with age and comprise between one fifth and one quarter of the combined ventricular output, proportions that remain unchanged through the second and third trimesters of pregnancy.