OBJECTIVE: To evaluate the influence of chorionicity on outcome in twin pregnancies with two live fetuses at the nuchal translucency scan in the first trimester.DESIGN: Population-based register study.SETTING: Denmark.POPULATION: A cohort of 3621 twin pregnancies, 84.3% dichorionic (DC) pregnancies and 15.7% monochorionic (MC) pregnancies (15% diamniotic [MCDA] and 0.7% monoamniotic [MCMA]).METHODS: A cohort study based on data collected from the Danish Fetal Medicine Database from July 2008 to July 2011.MAIN OUTCOME MEASURES: Fetal loss rate before week 22, intrauterine death rate from week 22 to birth, and neonatal mortality.RESULTS: Significantly more DC than MC pregnancies resulted in at least one live born infant-98.2% versus 92.3% in MCDA (P <0.05) and 66.7% in MCMA pregnancies (P <0.05). The rates of spontaneous loss of both fetuses before week 22 were 0.9, 2.4, and 20.8% for DC, MCDA and MCMA twins, respectively (P <0.05). The rate of intrauterine death of one fetus after week 22 was higher in MCDA twins than DC twins 1.7% versus 0.6% (P <0.05). We found no significant difference in the rate of neonatal death.CONCLUSIONS: The risk of spontaneous loss of both fetuses before week 22 of gestation was significantly higher in MCMA and MCDA pregnancies than in DC pregnancies, and significantly higher in MCMA pregnancies than in MCDA pregnancies. After week 22 the risk of loss of one fetus after week 22 was significantly higher in MCDA pregnancies than in DC pregnancies.