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Twenty healthy parturients in active labor were monitored with continuous wave Doppler to assess changes in uterine and umbilical velocity waveforms. Each case served as its own control. Tracings of fetal heart rate monitoring were normal in all patients. The analysis of the waveforms included the peak systolic/end-diastolic ratio for the umbilical circulation (umbilical artery), and the systolic minus diastolic velocity divided by systolic velocity (resistance index) was used as an indication of downstream resistance in the uterine arteries. Recordings from umbilical, left and right uterine arteries were obtained during various stages of progression of labor as indicated by Friedman's curve. In latent phase labor with intact membranes, as well as in three consecutive measurements throughout active phase labor until delivery, umbilical artery systolic/end-diastolic ratios, before, during and after contraction did not change--2.2 +/- 0.5, 2.3 +/- 0.6, 2.2 +/- 0.3 and 2.5 +/- 0.7 (NS). No changes in the resistance to flow in the left and right uterine arteries were recorded during both latent and active phases of labor--0.53 +/- 0.09, 0.52 +/- 0.1, 0.5 +/- 0.07, 0.56 +/- 0.07 (NS) and 0.59 +/- 0.1, 0.57 +/- 0.1, 0.56 +/- 0.1, 0.59 +/- 0.08 (NS), respectively. These results suggest stability of the fetal cardiovascular system ensuring continuous constant gas exchange process during labor, enabling most term fetuses to tolerate labor to a degree where minimal if any metabolic changes occur.