To investigate the effects of antenatal betamethasone on fetal pulmonary blood flow velocity waveforms. The study comprised 28 women with singleton pregnancies at high risk for preterm delivery. They were treated with two doses of 12 mg betamethasone intramuscularly 24 h apart to enhance lung maturity. Flow velocity waveforms were recorded with Doppler ultrasound from the middle segment of pulmonary artery (PA). Compared with the pretreatment mean value, a significant decrease in the pulmonary artery pulsatility (PI) and the resistance indexes (RI) was noted at 24 h and 48 h after the administration of first dose of betamethasone (p = 0.022 and p = 0.018 for PI and p = 0.001 and p = 0.004 for RI, respectively). After 7 days, the pulmonary artery velocity waveforms returned to the types of waveform observed before treatment (p = 0.216 for PI and p = 0.249 for RI). Maternal antenatal betamethasone resulted in a significant transient decrease in the pulsatility and the resistance indexes in the pulmonary artery. These findings indicate a direct effect of betamethasone on fetal pulmonary circulation.