Aim: To identify the threshold of monochorionic twins with amniotic fluid discordance (TAFD) below 26 weeks for predicting progression to adverse outcomes. Method: A retrospective study involving 68 women of monochorionic twins that do not meet fetoscope laser photocoagulation criteria was conducted. The TAFD was calculated as the vertical pocket of Twin 1 minus that of Twin 2, and the maximum TAFD in any period from 14 weeks to below 26 weeks was identified. We then calculated the ratio of the vertical pocket of Twin 1 to that of Twin 2 as the maximum TAFD ratio. We attempted to elucidate the cut-off value of the maximum TAFD ratio for adverse outcomes including cases that progressed to twin-twin transfusion syndrome (TTTS) after 26 weeks, twin anemia-polycythemia sequence, or neurologic abnormalities. Results: There were 21 cases of selective intrauterine growth restriction (sIUGR), 4 cases of twins that developed TTTS, one case of twins that developed TTTS with neurologic abnormalities, and 4 cases of twins that developed neurologic abnormalities. The median maximum TAFD ratio of the study group was 1.5. ROC curve analysis showed that a maximum TAFD ratio of 1.9 was the optimal cut-off value. In cases where the maximum TAFD ratio was greater than 1.9, the odds ratio for adverse outcomes was 15.4 when considering the presence of sIUGR. All cases of twins with neurological abnormalities had a maximum TAFD ratio greater than 1.9. Conclusion: Maximum TAFD ratio greater than 1.9 below 26 weeks increased adverse outcomes in monochorionic twins.