Objective: To evaluate the effect of antenatal betamethasone on fetal branch pulmonary artery flow velocity waveforms. Methods: Betamethasone 12 mg i.m. in 2 injections 24 h apart was given to 25 pregnant women at risk of preterm delivery. Pulsatility (PI) and resistance (RI) indexes were measured at the proximal, middle and distal segments of the fetal pulmonary artery before and repeatedly 1, 2 and 7 days after the first dose. ANOVA for repeated measures and the general linear model were used to statistically analyze the results. Results: Blood flow resistance decreased after betamethasone treatment at both the middle (p = 0.001 and p = 0.02 for RI and PI, respectively) and the proximal (p = 0.03 for RI) segments returning to basal values after 7 days. Conclusions: Antenatal betamethasone decreases fetal branch pulmonary artery vascular resistance.