The transition to extra-uterine life at birth represents a remarkable physiological challenge, particularly in pulmonary physiology as the lungs must immediately take over the role of gas exchange at birth. The extent of the change in pulmonary physiology that must occur at birth in order for the lungs to transform into an efficient gas exchange organ is exemplified by change in the pulmonary circulation. Throughout gestation, pulmonary vascular resistance (PVR) is high and the majority of right ventricular output bypasses the lungs and passes through the ductus arteriosus into the systemic circulation. As a result, fetal pulmonary blood flow (PBF) is low, but rapidly increases at birth due to a very large reduction in PVR, which also results in a gradual reduction in pulmonary arterial pressure. Despite the physiological significance of maintaining a high PVR during fetal development and of the large and rapid reduction in PVR at birth, the precise mechanisms controlling perinatal pulmonary haemodynamics are not fully understood. In particular, the role of intra-luminal pressure in regulating pulmonary haemodynamics in the immature lung before and after birth has received relatively little attention, although it is known to have a major impact in the adult lung. In this review, our primary aims are to discuss the importance of intra-luminal pressure in the maintenance and control of pulmonary haemodynamics both in utero, and postnatally, in preterm sheep. This review examines both chronic and phasic alterations in intra-luminal pressure, and the subsequent influence on pulmonary haemodynamics. Although very little data is available on preterm human fetus and infants, it is highly likely that the concepts discussed in this review are directly applicable to humans.