The successful transition from fetal to neonatal life is a major physiological challenge that requires the coordination of lung developmental processes, which culminate with the formation of a diffusible alveolar-capillary barrier, adequate pulmonary vasoreactivity, mature surfactant system, and clearance of lung fluid. During fetal life, gas exchange does not take place in fetal lungs but in the placenta. High pulmonary vascular resistance diverts blood flow to the left atrium through the foramen ovale and to the aorta via the ductus arteriosus. The placental circulation receives 30-50 % of the fetal cardiac output and is the major source of venous return to the fetal heart. Therefore, the umbilical venous return determines the preload for the left ventricle. Shortly before birth and during labor, the lungs undergo important transitional changes. The reabsorption of lung fluid within the airways is initiated during labor by adrenaline-induced activation of sodium channels. Uterine contractions during labor and the onset of inspiration after umbilical cord clamping generate a high transpulmonary pressure gradient leading to additional clearance of fluid from the airways into the surrounding tissue . Following the first breath and lung aeration, oxygen-induced vasodilation leads to a sudden rise in pulmonary blood flow and left atrial pressures, which closes the foramen ovale. Meanwhile, systemic vascular resistance increases above the level of pulmonary vascular resistance after placental removal, which reverses blood flow across the ductus arteriosus and induces ductal closure in response to high oxygen tension.

Premature birth can impact the success of adaptation to extrauterine life. Moderately preterm and late preterm births represented 4.4% of singleton live births in the Brussels area in 2020. Although they may be close to term, the loss of the last 4 to 8 weeks of gestation is vital to their physiologic and metabolic maturity. Because of their physiologic and metabolic immaturity, they have higher morbidity and mortality rates compared with term infants (gestational age 37 weeks). Although they may look similar to full-term infants, especially for the late preterm, the gap in the last few weeks of gestation is critical for physiological and metabolic maturation. Moderate and late preterm infants are at higher risk than term infants for a number of neonatal complications. This includes respiratory distress requiring non invasive or invasive ventilation, transient tachypnea of the newborn, intraventricular hemorrhage, periventricular leukomalacia, bacterial sepsis, apnoea, hypoglycemia, temperature instability, jaundice and hyperbilirubinaemia, feeding difficulties, neonatal intensive care admission, and also death. By contrast with lung's full-term newborn, lung of the preterm newborn presents an inability to adapt to extra-uterine life. Lung development at this time of gestation is in the saccular stage. Because of this immature lung structure, it results in delayed intrapulmonary fluid absorption, surfactant deficiency and inefficient gas exchange leading to respiratory morbidities such as transient tachypnea of the newborn, respiratory distress syndrome, persistent pulmonary hypertension. In addition, synchronicity and breath control is also immature and leads to apnea. These newborns exhibit a higher risk of positive pressure ventilation resuscitation at birth, admission to the neonatal intensive care unit (NICU), and severe hypoxic respiratory failure requiring mechanical ventilation in the most severe cases. In addition to increased neonatal morbidity, moderate or late preterm birth can impact mother-infant relationship. After delivery, immediate skin-to-skin contact during the first minute after birth is the natural process recommended to support mother-infant bonding and promote early onset of breastfeeding. Despite efforts made to start skin-to-skin contact as early as possible after delivery, immediate contact is practically difficult to implement related to the need for respiratory support for most of these newborns with incomplete transition to extrauterine life. In our institution, the infant is usually separated from the mother after umbilical cord clamping to provide first care by a pediatrician before returning on the mother's chest or on the father/partner's chest depending on parental wishes and maternal well-being during the operation and only if the condition of the newborn allows it. The separation between the mother and her newborn can be further extended in the case of NICU admission for various and multiple reasons related to prematurity.

The timing of umbilical cord clamping can profoundly affect the process of neonatal cardiorespiratory transition. Immediate cord clamping reduces the venous return to the heart, which transiently decreases heartbeats, cardiac output and cerebral blood flow before respiration initiates and pulmonary blood flow increases. Delayed cord clamping for longer than 60 seconds improves the transfusion of blood from the placenta to the newborn. Moreover, it can increase neonatal hemoglobin levels, improve long-term iron stores, and improve neurodevelopmental outcomes. Nevertheless, in both clinical research setting and daily practice, delayed cord clamping lasts rarely more than one minute during cesarean section. More recently, another approach, referred to as physiologically based cord clamping (PBCC), has been proposed to delay cord clamping up to 5 minutes after the onset of ventilation. PBCC allows to start lung aeration while on placental support and, therefore, promotes hemodynamic transition by increasing pulmonary blood flow and maintaining left ventricle preload. This strategy has been demonstrated efficient in preterm lambs and is feasible in very preterm infants, via the use of a purpose-designed resuscitation table that allows delayed cord clamping, maintenance of body temperature, and concomitant respiratory support where necessary. First experience has reported good parental acceptance of the procedure. Because PBCC has not been reported in moderate and late preterm infants, the present project aims to assess whether PBCC in moderate and late preterm infants would not be inferior to standard umbilical cord clamping with regards to adaptation to extrauterine life, respiratory morbidity, quality of mother-infant bonding, and maternal safety.