The ductus arteriosus (DA) is probably the most intriguing vessel in postnatal hemodynamic transition. DA patency in utero is an active state, in which prostaglandin E2 (PGE2) and nitric monoxide (NO), play an important role. Since the DA gets programmed for postnatal closure as gestation advances, in preterm infants the DA frequently remains patent (PDA). PGE2 exposure programs functional postnatal closure by inducing gene expression of ion channels and phosphodiesterases and anatomical closure by inducing intimal thickening. Postnatally, oxygen inhibits potassium and activates calcium channels, which ultimately leads to a rise in intracellular calcium concentration consequently inducing phosphorylation of the myosin light chain and thereby vasoconstriction of the DA. Since ion channel expression is lower in preterm infants, oxygen induced functional vasoconstriction is attenuated in comparison with full term newborns. Furthermore, the preterm DA is more sensitive to both PGE2 and NO compared to the term DA pushing the balance toward less constriction. In this review we explain the physiology of DA patency in utero and subsequent postnatal functional closure. We will focus on the pathobiology of PDA in preterm infants and the (un)intended effect of antenatal exposure to medication on both fetal and neonatal DA vascular tone.