Background Obstetric anal sphincter injuries lead frequently to short‐ and long‐term consequences for the mother, including perineal pain, genital prolapse, and sexual problems. The aim of the study was to evaluate whether the implementation of an intervention program in the second stage of labor involving altered perineal support techniques reduced severe perineal trauma. Methods All women reaching the second stage of labor and giving birth vaginally to singleton babies at Landspítali University Hospital (comprising 76% of births in Iceland in 2013) were enrolled in a cohort study. Data were recorded retrospectively for 2008‐2010 and prospectively in 2012‐2014, for a total of 16 336 births. During 2011, an intervention program was implemented, involving all midwives and obstetricians working in the labor wards. Two professionals assessed and agreed on classification of every perineal tear. Results The prevalence of obstetric anal sphincter injuries decreased from 5.9% to 3.7% after the implementation (P < 0.001). Third‐degree tears decreased by 40%, and fourth‐degree tears decreased by 56% (P < 0.001). The prevalence of first‐degree tears increased from 25.8% to 33.1%, whereas second‐degree tears decreased from 44.7% to 36.6% between the before and after study periods. Severe perineal trauma was linked to birthweight, and this did not change despite the new intervention. Conclusions Active intervention to reduce perineal trauma was associated with an overall significant decrease in obstetric anal sphincter injuries. Good perineal visualization, manual perineal support, and controlled delivery of the fetal head were essential components for reducing perineal trauma.