Steady-state wave intensity (WI) analysis indicates that characteristic mid-systolic falls in fetal pulmonary trunk (PT) and artery (PA) blood flow are due to a large backward-running compression wave (BCW_ms) that 1) originates from the pulmonary microvasculature by a combination of cyclical pulmonary vasoconstriction and vascular reflection of the forward-running compression wave (FCW_is) associated with impulsive right ventricular ejection, and 2) is transmitted into the PT. However, no information is available about the dynamic properties of this PA BCW_ms and its contribution to beat-to-beat regulation of pulmonary hemodynamics. Accordingly, beat-by-beat WI analysis was performed during brief increases in ventricular contractility accompanying an extrasystole (ES) in nine anesthetized late-gestation fetal sheep instrumented with PT and left PA micromanometer catheters to measure pressure (P) and transit-time flow probes to obtain blood velocity (U). WI was calculated as the product of P and U rates of change. At steady-state, the magnitude of PA BCW_ms, and its associated P and U changes (P and U respectively), were similar to those of FCW_is. The PA FCW_is and BCW_ms, and their accompanying P and U, were all transiently potentiated after an ES. Beat-by-beat PA FCW_is-BCW_ms wave area, P and U relationships were highly linear (R²0.91) with slopes of 1.36-1.47 (P<0.001), consistent with the presence of a vasoconstrictor component in PA BCW_ms. PA-PT BCW_ms area, P and U relationships were also linear (R²0.77) with slopes of 0.23-0.64 (P<0.001). These results indicate that the fetal PA BCW_ms contributes to beat-to-beat regulation of not only PA but also PT hemodynamics.