This study aims to identify the effect of third interstitial fluid on adverse outcomes in twin pregnancies with severe pre-eclampsia, and explore the differences in bad ending between twins and singletons.The present retrospective cohort study was conducted on patients with severe pre-eclampsia, who delivered in Tongji Hospital, Wuhan, China, between 2017 and 2022. The adverse outcomes in singleton and twin pregnancies with severe pre-eclampsia were initially investigated. Then, the diverse maternal and fetal consequences between singleton and twin pregnancies in patients with severe pre-eclampsia were compared after merging with the third interstitial fluid.A total of 709 patients were included for the present study. Among these patients, 68 patients had twin pregnancies, and 641 patients had singleton pregnancies. The rate of postpartum hemorrhage (2.81% vs. 13.24%, P<0.001), and admission rate to the Neonatal Intensive Care Unit (NICU) after birth (30.73% vs. 63.24%, P=0.011) were significantly higher in twin pregnancies. The neonatal weight of twins was statistically lower than singletons (1964.73±510.61 g vs. 2142.92±731.25 g, P=0.008). For the groups with the third interstitial fluid, the delivery week (P=0.001) and rate of admission to the NICU after birth were significantly advanced in twin pregnancy group, when compared to singleton pregnancy group (P=0.032), and the length of hospital stay was shorter (P=0.044). Furthermore, there was no statistically significant difference between the twin pregnancy group and the singletony pregnancy group without the third interstitial fluid.The maternal and fetal adverse outcomes of patients with severe pre-eclampsia increased in twin pregnancies, when compared to singleton pregnancies. Thus, when patients develop the third interstitial fluid, twin pregnancies would more likely lead to adverse fetal outcomes, when compared to singleton pregnancies, and there would be no significant difference in maternal adverse outcomes. More attention should be given to patients who merge with the third interstitial fluid.© 2023. Huazhong University of Science and Technology.