ObjectiveTo identify the source of umbilical venous velocity pulsations, times of transmission from the atrial contraction pressure waveform to velocity waves in the inferior vena cava, ductus venosus, intra-abdominal umbilical vein, and intra-amniotic umbilical vein were examined.MethodsFive lamb fetuses at 125-135 days's gestation were instrumented with solid satte pressure transducers in the inferior vena cava, fluid-filled catheters in the inferior vena cava and descending aorta, ad epicardial pacemakers. Three to 5 days postoperatively, inferior vena cava, ductus venosus, and umbilical vein velocities were examined with Doppler ultrasoound. Normal saline was administered until umbilical vein velocity pulsations developed (180 ± 60 mL). In three fetuses, premature atrial contractions were induced under baseline conditions and after umbilical vein velocity pulsations developed.ResultsTimes of transmission from the atrial contraction pressure waveform until velocity decreases in the fetal venous system were significantly different in the inferior vena cava, ductus venosus, intra-abdominal umbilical vein, and intra-amniotic umbilical vein (P< .001). Times increased with the distnance from the atrium. Inferior vena cava pressure increased with fluid administration from 3.7 ± 4.7 mmHg to 9.3 ± 2.3 mmHg (P< .01). Time from increased pressure waveforms with induced premature atrial contractions to the nadir of subsequent umbilical vein velocity waves decreased from 0.123 ± 0.047 seconds before saline administration to 0.072 ± 0.039 seconds after saline administration (P< .001).ConclusionTransmission time of atrial pressure into the venous circulation increases with distance from the atrum and decreases with volume loading. Umbilical venous velocity pulsations derive from atrial pressure changes transmitted in a retrograde fashion.