Fetal cardiac function was studied in 52 women at 10-20 weeks of normal gestation using Doppler ultrasonography. According to a cross-sectional study design, transmitral and ascending aortic flow velocity waveforms were obtained simultaneously, in order to calculate filling and ejection time as well as isovolumic contraction and relaxation time. A statistically significant gestational age-dependent increase was established for both transmitral and ascending aortic flow velocities. Cardiac cycle length and filling time displayed a statistically significant increase with advancing gestational age, whereas ejection time and isovolumic relaxation time showed a gestational age-related decrease. No relationship existed between isovolumic contraction time and gestational age. A statistically significant decrease was demonstrated for atrial contribution to ventricular filling at 10-14 weeks of gestation, with a constant pattern during the remainder of the study period. No relationship existed between the different components of the cardiac cycle and mitral and aortic flow velocity parameters. This study shows that the late first and early second trimesters of pregnancy are characterized by gestational age-related changes in fetal cardiac function.