OBJECTIVE: To establish reference intervals for fetal heart stroke volume and cardiac output with gestation.METHODS: Fetal heart ventricular volumes were measured using the four-dimensional (4D) spatiotemporal imaging correlation (STIC) ultrasound technique in 140 normal singleton pregnancies at 12-34 weeks' gestation. The Virtual Organ Computer-aided AnaLysis (VOCAL) technique was used to obtain a sequence of six sections of each ventricular volume in systole and diastole. Each volume was obtained after a 30 degrees rotation from the previous one around a fixed axis extending from the apex of the heart to the point that divides symmetrically each atrioventricular valve. The contour of each ventricle was drawn manually and the 4D volumes of the left and right ventricle in systole and diastole were estimated. The stroke volume for each ventricle was then calculated by subtracting the one in systole from the one in diastole and the cardiac output was calculated by multiplying the stroke volume by the fetal heart rate. In 50 cases the stroke volumes were measured by the same sonographer twice and the intraobserver agreement of measurements was calculated.RESULTS: The left and right stroke volume and cardiac output increased exponentially with gestation, from respective mean values of 0.02 mL, 0.01 mL, 2.39 mL/min and 1.80 mL/min at 12 weeks to 0.30 mL, 0.32 mL, 43.46 mL/min and 46.72 mL/min at 20 weeks, and 2.07 mL, 2.67 mL, 284.71 mL/min and 365.99 mL/min at 34 weeks. The ratio of right to left stroke volume increased significantly with gestation from about 0.97 at 12 weeks to 1.13 at 34 weeks. In the Bland-Altman test, the mean percentage difference and 95% limits of intraobserver agreement for left stroke volume and right stroke volume were - 2.1 (-18.4, 14.2)% and - 0.8 (-16.4, 18.0)%, respectively.CONCLUSIONS: In normal fetuses the stroke volume and cardiac output increase between 12 and 34 weeks' gestation. The extent to which, in pathological pregnancies, possible deviations in these measurements from normal prove to be useful in the prediction of outcome remains to be determined.