Incidence and Risk Factors of Third- and Fourth-Degree Perineal Tears in Vaginal Delivery: A Retrospective Study at Bint Al-Huda Teaching Hospital, Nasiriyah, Iraq

Abstract

Obstetric anal sphincter injuries (OASIS) — third- and fourth-degree perineal tears — are an important cause of maternal morbidity, including anal incontinence, perineal pain, dyspareunia, and psychological distress. Their reported incidence varies widely (0.6–19.3%) with detection practices and case-mix. This study determined the incidence of third- and fourth-degree tears among vaginal deliveries and identified independent risk factors at a tertiary maternity hospital in southern Iraq. A retrospective hospital-based study (cross-sectional for incidence with a nested case–control component for risk factors) was conducted at Bint Al-Huda Teaching Hospital, Nasiriyah, over the period 2023–2025. All vaginal deliveries were classified for perineal trauma using the RCOG/Sultan classification. Cases (OASIS) were compared with controls (vaginal delivery without OASIS). Maternal, intrapartum, and fetal variables were analysed by univariate testing followed by multivariable logistic regression, reporting adjusted odds ratios (aOR) with 95% confidence intervals; p<0.05 was significant. Reporting followed STROBE. Among 3,000 vaginal deliveries, OASIS occurred in 150 (5.0%) — 126 (4.2%) third-degree and 24 (0.8%) fourth-degree. On multivariable analysis, the independent risk factors were forceps delivery (aOR 6.20), nulliparity (aOR 5.80), occipito-posterior position (aOR 2.44), birthweight >4 kg (aOR 2.30), prolonged second stage (aOR 2.00), shoulder dystocia (aOR 1.90), and oxytocin augmentation (aOR 1.70); epidural analgesia was not independently associated. OASIS rates rose steeply with operative delivery (3.0% spontaneous, 9.5% vacuum, 16.0% forceps). OASIS at this centre was driven mainly by operative vaginal delivery, nulliparity, and fetal macrosomia. Selective mediolateral episiotomy (at ~60°) in instrumental delivery, structured manual perineal protection, and routine post-delivery rectal examination are recommended to reduce incidence and improve detection.