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In many high-income countries, severe perinatal trauma (SPT), affecting the anal sphincter muscle complex, has been on the rise over the last decade. However, effective strategies to prevent SPT are scarce. In randomized controlled trials, only perineal warm compresses and massage have shown benefit. Other cohort studies have shown that prevention models involving several components can decrease the occurrence of SPT, including a stepped-wedge design that reduced SPT from 3.3% to 3.0%. In Scandinavia, a preventive strategy called collegial assistance has been used to help prevent SPT. This strategy involves 2 mid-wives who assist the woman in the second stage of labor, with the second midwife primarily focused on preventing SPT. The aim of this study was to compare the rate of SPT in pregnancies managed by collegial assistance versus a single midwife. This Oneplus study was a randomized, controlled, unmasked trial, conducted at 5 obstetric units in Sweden between December 10, 2018, and March 21, 2020. Included were adult women with uncomplicated singleton pregnancies at >37 weeks of gestation, who were carrying their first child or having their first vaginal birth after cesarean delivery. Excluded were women who had multiple pregnancies, had intrauterine fetal demise, were undergoing a planned cesarean section, or were at <37 weeks' gestation. Women were randomly assigned to either the intervention group with 2 midwives in attendance during active second stage labor or standard care with 1 midwife. All midwives were asked to document the preventive methods used in case report forms. A total of 3750 women were included in the final analysis-with 1879 women receiving collegial assistance and 1871 women receiving standard care. Of the women who gave birth spontaneously, 1546 were in the intervention group and 1513 in the standard care group. Severe perinatal trauma occurred less frequently in the intervention group than the standard care group (3.9% vs 5.7%; odds ratio, 0.68; 95% confidence interval, 0.49-0.97; P = 0.025). In the intervention group, 0.2% had fourth-degree tears compared with 0.5% in the standard care group. The median time for collegial assistance was 15 minutes (interquartile range, 10-20 minutes). The use of perineal warm compresses was similar in the intervention group and standard care group (86.4% vs 85.7%, respectively). No differences were observed in birth positions, manual perineal protection, neonatal outcomes, or secondary maternal outcomes. In conclusion, the attendance of a second midwife dedicated to preventing SPT during the second stage of labor significantly reduced the risk of injury.