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Transition from fetal to extrauterine circulation.

In utero, there is increased pulmonary vascular resistance favoring minimal pulmonary blood flow and right-to-left shunting of blood through the ductus arteriosus and foramen ovale. Delivery initiates a series of complex biochemical and structural changes whereby the neonate assumes those processes critical to survival. This article reviews fetal circulation, the characteristics of the fetal cardiopulmonary system, and the transition from fetal to neonatal circulation. It also discusses the consequences of disrupted transition to extrauterine life and the relationship between fetal cardiopulmonary anatomy and congenital heart disease.