- Midline episiotomy is a known major risk factor for severe perineal lacerations. The study was aimed to define obstetric risk factors for third-degree perineal tears in a university medical center where midline episiotomies are not performed. A comparison between vaginal deliveries complicated with third-degree perineal tears and deliveries without third-degree perineal tears was performed. Deliveries occurred between the years 1988-1999 in a tertiary medical center. Multiple gestations, preterm deliveries (<37 weeks' gestation), cesarean deliveries and cases of shoulder dystocia were excluded from the analysis. A multiple logistic regression model was constructed in order to find independent risk factors for third-degree perineal tears. Odds ratios (OR) and their 95% confidence interval (CI) were calculated from the regression coefficient. During the study period, 79 (0.1%) consecutive cases of third degree perineal tears were identified. Significant risk factors from the univariate analysis were fetal macrosomia (OR 2.7, 95% CI 1.2-5.5), nulliparity (OR 2.9, 95% CI 1.8-4.6), labor induction (OR 1.9, 95% CI 1.0-3.5), failure of labor to progress during the second stage (OR 10.8, 95% CI 5.4-21.1), non-reassuring fetal heart rate patterns (OR 11.7, 95% CI 6.1-21.5), mediolateral episiotomy (OR 2.8, 95% CI 1.8-4.5), vacuum extraction (OR 10.6, 95% CI 6.1-18.3), and forceps delivery (OR 29.2, 95% CI 7.3-97.2). However, using a multivariable analysis, only fetal macrosomia (OR 2.5, 95% CI 1.2-4.9), vacuum extraction (OR 8.2, 95% CI 4.7-14.5), and forceps delivery (OR 26.7, 95% CI 8.0-88.5) remained as independent risk factors. The combined risk for instrumental deliveries of macrosomic newborns was 8.6 (95% CI 1.2-62.5; p=0.010). After adjustment for possible confounding variables, mediolateral episiotomy per se was not an independent risk factor for third-degree perineal tears. Instrumental vaginal deliveries of macrosomic fetuses should be avoided whenever possible to decrease the occurrence of third-degree perineal tears.