This was a prospective cohort study. During March 2018–March 2019, 27 singleton pregnancies between 28 and 34 gestational weeks with a diagnosis threat of preterm birth were included prospectively. To be eligible, patients had to have received betamethasone and a loading dose of nifedipine for tocolysis, followed by hydration and maintenance nifedipine therapy for 48 h. Betamethasone (Celestone Cronodose, Schering-Plough, İstanbul, Turkey) was administered as two doses of 12 mg each, administered 24 h apart. Fetuses, who had intrauterine growth restriction (IUGR), oligohydramnios, polyhydramnios, or fetal anomaly were excluded from the study. Other maternal indications for exclusion were: preeclampsia, eclampsia, HELLP syndrome, premature preterm rupture of membranes, placenta previa, or any chronic maternal disease. Patients with diabetes mellitus (gestational or pregestational) were not included in the study. We prospectively evaluated fetal main pulmonary artery (MPA), left and right pulmonary artery (PA), left myocardial performance index (MPI, TEI index), tricuspid valve