The transition from fetal to neonatal circulation requires a concert of events to transfer gas exchange function from the placenta to the lungs and separate the pulmonary and systemic pathways. Pulmonary vascular resistance (PVR) rapidly decreases within the first minutes of extrauterine life and continues to gradually decrease during the first week, increasing pulmonary blood flow and reducing pulmonary pressure [1, 2]. Umbilical vessels constrict, removing the placental circulation and leading to closure of the ductus venosus (DV) [2]. The increased left atrial filling and reduced right atrial filling results in permanent closure of the flap of the foramen ovale, removing the R→L interatrial shunt. Closure of the ductus arteriosus (DA) completes the separation of the pulmonary and systemic circulations by 48 hours in 82% of term newborns and by 96 hours in 100% [3]. Removal of the placental circulation is routinely achieved by umbilical cord clamping (UCC) immediately after birth. This practice, however, has been called into question by many studies, which suggest that continued umbilical flow in the early neonate is beneficial, and immediate UCC can lead to infant anemia [4, 5]. Due to routine UCC, the effects of this practice on transitional flow patterns are largely unknown [1, 6]. We therefore developed a lumped parameter model (LPM) to study the role of UCC in the fetal to neonatal transition. Our model includes time-varying resistance functions that allow us to simulate the opening of the PVR and closure of the DA and umbilical vessels. This model demonstrates that UCC can lead to an earlier onset of DA flow reversal and slightly reduced cardiac output (CO).
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