AbstractBackground:There is a paucity of functional data on mid-to-late preterm infants between 30⁺⁰ and 34⁺⁶ weeks gestation. We aimed to characterise transitional cardiopulmonary and haemodynamic changes during the first 48 hours in asymptomatic mid-to-late preterm infants.Methods:Forty-five healthy preterm newborns (mean ± standard deviation) gestation of 32.7 ± 1.2 weeks) underwent echocardiography on Days 1 and 2. Ventricular mechanics were assessed by speckle tracking-derived deformation, rotational mechanics, tissue Doppler imaging, and right ventricle-focused measures (tricuspid annular plane systolic excursion, fractional area change). Continuous haemodynamics were assessed using the NICOM™ system to obtain left ventricular output, stroke volume, heart rate, and total peripheral resistance by non-invasive cardiac output monitoring.Results:Right ventricular function increased (all measures p < 0.005) with mostly stable left ventricular performance between Day 1 and Day 2. NICOM-derived left ventricular output [mean 34%, 95% confidence interval 21–47%] and stroke volume [29%, 16–42%] increased with no change in heart rate [5%, −2 to 12%]. There was a rise in mean blood pressure [11%, 1–21%], but a decline in total peripheral resistance [−14%, −25 to −3%].Conclusion:Left ventricular mechanics remained persevered in mid-to-late premature infants, but right ventricular function increased. Non-invasive cardiac output monitoring is feasible in preterm infants with an increase in left ventricular output driven by an improvement in stroke volume during the transitional period.