Indomethacin is a commonly used medication against preterm delivery. Several reports of fetal ductal constriction have been described after indomethacin use in the literature; however, there are no previously documented reports describing an association between Twin-Twin Transfusion syndrome and a constrictor effect of indomethacin on the ductus arteriosus. Two patients were referred to our department for Twin-Twin Transfusion syndrome and each underwent placental laser surgery. Constriction of the ductus arteriosus occurred as early as 20 and 24 weeks' gestation following maternal use of indomethacin after laser surgery. Spontaneous amelioration was observed after discontinuation of the drug. The constrictor effect of indomethacin on the ductus arteriosus can be observed even after a single dose and as early as 20 weeks of gestation in complicated monochorionic twin pregnancies. We emphasize meticulous use of indomethacin in complicated monochorionic twin pregnancies because the constrictive effect seems to be independent of gestational age.