The fetal circulation has two major vascular shunts, the ductus arteriosus and the ductus venosus. The ductus arteriosus connects the pulmonary artery with the descending portion of the aortic arch, hence shunting most of the right ventricular output away from the unexpanded lungs. The ductus venosus connects instead the portal sinus with the inferior vena cava and allows well-oxygenated umbilical vein blood to bypass the liver and reach the central circulation rapidly. Both blood vessels cease their function after birth and undergo permanent closure. It is now well established that prenatal patency of the ductus arteriosus is an active state sustained by a prostaglandin. A similar mechanism has been recently recognized in the fetal ductus venosus. Evidence is presented indicating that prostaglandin E2 and prostaglandin I2 are natural relaxants, respectively, for the ductus arteriosus and the ductus venosus. In addition, both vascular shunts share the dependence on an endogenous cytochrome P-450 mechanism to develop their contractile tone. This mechanism may be important in the normal process of shunt closure at birth. While broadening the knowledge of fetal cardiovascular homeostasis, advances in this field have important implications for the prevention and management of certain pathological conditions affecting the newborn.