The fetal circulation is a parallel circulation with an open PDA, an open foramen ovale, and very high pulmonary vascular resistance. At birth, the pulmonary vascular resistance drops, the patient ovale is functionally closed, and the patent ductus closes in response to oxygen. This final circulation is a series circulation. In the setting of hypoxia, hypothermia, acidosis, and congenital cardiac anomalies, the circulation reverts back to an ever-changing transitional circulation and back to a fetal circulation.