Abstract Introduction Perineal tears are classified into four degrees based on severity. Second‐degree tears, which involve the muscles fusing into the perineal body, but not the anal sphincter, vary in extent and have recently been subcategorized. However, knowledge about their long‐term anatomical impact on the perineal body remains limited, as postpartum evaluation has largely relied on inspection and palpation. This study aimed to examine the association between the severity of perineal tears not affecting the anal sphincter, diagnosed at delivery, and deviations in the muscles fusing into the perineal body, assessed by three‐dimensional ultrasound 1 year after vaginal delivery. Material and Methods This prospective cohort study included 384 primiparous women with vaginal deliveries at Akershus University Hospital, Norway, from January 2021 to July 2022. Perineal tears were classified and sutured according to international standards at delivery. In addition, second‐degree tears were subclassified according to the percentage of the perineal body damage (<50% = 2A, ≥50% = 2B, entire perineal body without anal sphincter = 2C). Women with third‐ or fourth‐degree tears were excluded. One year after delivery, participants underwent three‐dimensional endovaginal and endoanal ultrasound examinations. The transverse perineal, puboperinealis, and puboanalis muscles were examined at their fusion into the perineal body. A deviation was defined as a muscle discontinuity or absence in at least one of the muscles. Results The proportion of women with sonographically detected muscle deviations increased with tear severity: 22% for first‐degree ( n = 24), 52.7% for 2A ( n = 29), 60% for 2B ( n = 21) and 86.2% for 2C ( n = 25). Among women with episiotomies ( n = 126), 60.6% had one or more muscle deviations. A higher number of muscles were affected in the 2C tear group compared to 2B and 2A. Conclusion Sonographic deviations in the muscles fusing into the perineal body increase with the severity of perineal tears.