BACKGROUND: Monochorionic twin pregnancies complicated by severe twin to twin transfusion syndrome at midgestation can be treated by either serial amnioreduction(removal of large volumes of amniotic fluid) or selective fetoscopic laser coagulation of the communicating vessels on the chorionic plate. We conducted a randomized trial to compare the efficacy and safety of these two treatments. METHODS: Pregnant women with severe twin to twin transfusion syndrome before 26 weeks of gestation were randomly assigned to laser therapy or amnioreduction. We assessed perinatal survival of at least one twin (a prespecified primary outcome),survival of at least one twin at six months of age, and survival without neurologic complications at six months of age on the basis of the number of pregnancies or the number of fetuses or infants, as appropriate. RESULTS:The study was concluded early, after 72 women had been assigned to the laser group and 70 to the amnioreduction group, because a planned interim analysis demonstrated a significant benefit in the laser group. As compared with the amnioreduction group, the laser group had a higher likelihood of the survival of at least one twin to 28 days of age (76 percent vs. 56 percent; relative risk of the death of both fetuses, 0.63; 95 percent confidence interval, 0.25to 0.93; P=0.009) and 6 months of age (P=0.002). Infants in the laser group also had a lower incidence of cystic periventricular leukomalacia (6 percent vs. 14 percent,P=0.02) and were more likely to be free of neurologic complications at six months of age (52 percent vs. 31 percent, P=0.003). CONCLUSIONS: Endoscopic laser coagulation of anastomoses is a more effective first linetreatment than serial amnioreduction for severe twin totwin transfusion syndrome diagnosed before 26 weeks of gestation. BACKGROUND: Monochorionic twin pregnancies complicated by severe twin to twin transfusion syndrome at midgestation can be treated by either serial amnioreduction (removal of large volumes of amniotic fluid) or selective fetoscope laser coagulation of the communications vessels on the chorionic plate. We conducted a randomized trial METHODS: Pregnant women with severe twin to twin transfusion syndrome before 26 weeks of gestation were randomly assigned to laser therapy or amnioreduction. We assessed perinatal survival of at least one twin (a prespecified primary outcome ), survival of at least one twin at six months of age, and survival without neurologic complications at six months of age on the basis of the number of pregnancies or the number of fetuses or infants, as appropriate. RESULTS: The study was early early , after 72 women had been assigned to the laser group and 70 to the amnioreduction group, because a planned i nterim analysis demonstrated a significant benefit in the laser group. the compared with the amnioreduction group, the laser group had a higher likelihood of the survival of at least one twin to 28 days of age (76 percent vs. 56 percent; relative risk of the 95% confidence interval, 0.25 to 0.93; P = 0.009) and 6 months of age (P = 0.002). Infants in the laser group also had a lower incidence of cystic periventricular leukomalacia (6 percent vs. 14 percent, P = 0.02) and were more likely to be free of neurologic complications at six months of age (52 percent vs. 31 percent, P = 0.003). CONCLUSIONS: Endoscopic laser coagulation of anastomoses is a more effective first line treatment than serial amnioreduction for severe twin totwin transfusion syndrome diagnosed before 26 weeks of gestation.