Core concepts: Fetal cardiac physiology

The fetal myocardium and circulation differ from that of the adult in many important ways. Postnatal circulation occurs in series with the right ventricle providing a full cardiac output to the pulmonary circulation and the left ventricle delivering that same cardiac output to the body (systemic circulation). In the fetus, however, there is a parallel circulation in which organs receive blood flow from both ventricles, and the ventricular output is described as combined. Due to this arrangement, the fetus has unique methods to adapt to intrauterine stressors on the cardiovascular system. There are several normal physiologic transitions that take place after birth which may be perturbed by a compromised hemodynamic state or the presence of congenital heart disease. The transition to an adult circulation is a dynamic process, of which the understanding is critical to the care of neonates.