BACKGROUND: Perineal tears at delivery are common. The current WHO classification system compacts all the varying extents of second-degree tears into one code. Some tears lead to long-term injuries. The correct identification and classification of disease is necessary for correct clinical management as well as for research. Regulatory standards govern care practices. This article describes the process of creating and testing new subclassifications for second-degree tears at delivery. METHODS: The development and implementation of new subclassifications of second-degree perineal tears after delivery in Sweden are described. The new classification was tested for incidence and relevance via the national perineal laceration register (PLR) in 11,203 women with prospectively recorded second degree tears. RESULTS: Second-degree tears after delivery are subdivided into four subgroups according to the anovaginal distance and the extent in length and depth of the largest perineal/vaginal tear, which can be combined with uni-or bilateral levator ani avulsion. Women with larger second-degree tears were more likely than women with smaller tears to report complications after eight weeks (OR 1.41 CI 1.21-1.64, p <
0.001) and one year (OR 1.27, CI 1.1-1.46, p <
0.001). CONCLUSION: Detailed subclassifications of perineal and vaginal tears are implemented in the Swedish ICD-10 coding system and Swedish national registers. The outcomes after second-degree tears differ according to their extent, which corroborates the classification rationale. These subclassifications can be used in studies of preventive measures, treatment and patient-reported outcomes and experiences taking into account the extent of second-degree perineal tears at delivery. TRIAL REGISTRATION: Data regarding women were prospectively collected from the National perineal laceration register (PLR) from January 1, 2021, to December 31, 2022.