Perineal body length as a predictor of severe perineal lacerations in a Turkish obstetric cohort

This study aimed to investigate the association between perineal body length (PBL) and the occurrence of severe perineal trauma, defined as third- and fourth-degree lacerations, in a Turkish obstetric population. A prospective observational study was conducted at a tertiary maternity hospital between April 2024 and March 2025. A total of 495 women meeting predefined eligibility criteria were included. PBL was measured once during the first stage of labor at 4 cm cervical dilation using a sterile paper tape. Maternal demographic, obstetric, and neonatal characteristics were prospectively collected, and perineal trauma was classified according to the Royal College of Obstetricians and Gynecologists criteria. The primary outcome was the incidence of severe perineal trauma. Secondary analyses included ROC-based threshold identification, assessment of linearity using continuous PBL modeling, and internal validation of model performance. Severe perineal trauma occurred in 42 women (8.5%), more frequently in nulliparas than multiparas (10.4% vs. 7.2%). Women with OASIS had significantly shorter mean perineal body length compared with those without severe trauma (3.39 ± 0.44 cm vs. 3.88 ± 0.49 cm, p < 0.001). Multivariate analysis demonstrated that shorter PBL (OR 2.31, 95% CI 1.54-3.46), prolonged second stage of labor (OR 1.72, 95% CI 1.18-2.53), higher neonatal birth weight (OR 1.89, 95% CI 1.27-2.80), and larger head circumference (OR 2.04, 95% CI 1.32-3.15) were independent predictors of OASIS. Continuous modeling confirmed an approximately linear relationship between decreasing PBL and increasing OASIS risk (per 0.5-cm decrease: aOR 1.68). A candidate predictive threshold of < 3.5 cm was identified (AUC 0.78 for continuous PBL; NPV 95%). Shorter perineal body length is an independent and clinically relevant predictor of OASIS. Its strong negative predictive value and consistent performance across parity groups support its potential role as an adjunctive intrapartum risk marker for individualized intrapartum risk assessment. Given the prevalence-dependent nature of predictive values, PBL should not be used as a stand-alone screening tool, and the proposed threshold requires external validation. Routine assessment of PBL during labor may enable early identification of high-risk women and guide selective use of mediolateral episiotomy, perineal support, and other targeted preventive strategies to reduce severe perineal trauma.