A fetal blood vessel connecting the pulmonary artery with the descending aorta[MESH]. In the developing fetus, the ductus arteriosus is a shunt connecting the pulmonary artery to the aortic arch. It allows most of the blood from the right ventricle to bypass the fetus' fluid-filled lungs, protecting the lungs from being overworked and allowing the left ventricle to strengthen. There are two other fetal shunts, the ductus venosus and the foramen ovale[MP]. the fetal vessel that connects the left pulmonary artery with the descending aorta; the ductus arteriosus normally regresses into a fibrous cord, the ligamentum arteriousum after birth[MP] [ http://en.wikipedia.org/wiki/Ductus_arteriosus ]