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OBJECTIVES: Monoamniotic twin pregnancies are at increased risk of perinatal complications, primarily owing to the risk of cord entanglement. There is no recommendation on whether such pregnancies should be managed in hospital or can be safely managed in an outpatient setting, and the timing of planned delivery is also a subject of debate. The aim of this study was to compare the perinatal outcomes of inpatient vs outpatient fetal surveillance approaches employed among 22 participating study centers, and to calculate the fetal and neonatal death rates according to gestational age, in non-anomalous monoamniotic twins from 26 weeks' gestation. METHODS: The MONOMONO study was a multinational cohort study of consecutive women with monochorionic monoamniotic twin pregnancies, who were referred to 22 university hospitals in Italy, the USA, the UK and Spain, from January 2010 to January 2017. Only non-anomalous uncomplicated monoamniotic twin pregnancies with two live fetuses at 26 + 0 weeks' gestation were included in the study. In 10 of the centers, monoamniotic twins were managed routinely as inpatients, whereas in the other 12 centers they were managed routinely as outpatients. The primary outcome was intrauterine fetal death. We also planned to assess fetal and neonatal death rates according to gestational age per 1-week interval. Outcomes are presented as odds ratio (OR) with 95% CIs. The main outcome was analyzed using both standard logistic regression analysis, in which each fetus was treated as an independent unit, and a generalized mixed-model approach, with each twin pair treated as a cluster unit, considering that the outcome for a twin is not independent of that of its cotwin. RESULTS: 195 consecutive pregnant women with a non-anomalous uncomplicated monoamniotic twin gestation (390 fetuses) were included. Of these, 75 (38.5%) were managed as inpatients and 120 (61.5%) as outpatients. The overall perinatal loss rate was 10.8% (42/390) with a peak fetal death rate of 4.3% (15/348) occurring at 29 weeks' gestation. There was no significant difference in mean gestational age at delivery (31 weeks), birth weight (∼1.6 kg), or emergency delivery rate between the inpatient and outpatient surveillance groups. Based on generalized mixed-model analysis, there was no statistically significant difference in fetal death rates between inpatient management commencing from around 26 weeks compared with outpatient surveillance protocols from 30 weeks (3.3% vs 10.8%; adjusted OR 0.21 (95% CI, 0.04-1.17)). Maternal length of stay in the hospital was 42.1 days in the inpatient group, and 7.4 days in the outpatient group (mean difference 34.70 days (95% CI, 31.36-38.04 days). From 32 + 0 to 36 + 6 weeks, no fetal or neonatal death in either group was recorded. 46 fetuses were delivered after 34 + 0 weeks, and none of them died in utero or within the first 28 days postpartum. CONCLUSION: In uncomplicated monoamniotic twins, inpatient surveillance is associated with similar fetal mortality as outpatient management. After 31 + 6 weeks, and up to 36 + 6 weeks, there were no intrauterine fetal deaths or neonatal deaths. Copyright © 2018 ISUOG. Published by John Wiley & Sons Ltd.