PPHN occurs when a newborn's circulatory system does not adapt to breathing outside the womb. In the womb, a fetus's blood circulation bypasses the lungs through the ductus arteriosus and foramen ovale. After birth, these pathways should close as the lungs take over the role of oxygenating the blood. In PPHN, these pathways remain open, causing high pressure in the lungs and preventing adequate oxygenation.