Association of perineal length and thickness with spontaneous perineal tears during vaginal delivery: a prospective study.

OBJECTIVE To evaluate perineal length and thickness during labor and to assess their association with any spontaneous perineal tear. STUDY DESIGN This prospective study included 221 women with term singleton pregnancies and cephalic presentation. Perineal length and thickness were measured in early labor (≤4 cm cervical dilatation) and reassessed in the second stage (full dilatation). The primary outcome was any spontaneous perineal tear, including grades I-IV. Multivariable logistic regression analysis was performed to assess these associations, adjusting for parity, body mass index and birth weight. RESULTS Spontaneous perineal tears occurred in 31 women (14.0%). In multivariable analysis, shorter perineal length in the first stage of labor was independently associated with higher odds of spontaneous tear (OR 0.42; 95% CI 0.20-0.86; p = 0.018). In the second-stage model, greater perineal thickness was independently associated with higher odds of spontaneous tear (OR 2.34; 95% CI 1.08-5.05; p = 0.031). BMI, parity, and neonatal birth weight were not independently associated with the outcome. In sensitivity analyses with additional adjustment for episiotomy, perineal morphometric characteristics were no longer statistically significant. CONCLUSION Shorter perineal length in early labor and greater perineal thickness at full dilatation were associated with any spontaneous perineal tear in primary multivariable analyses. However, after additional adjustment for episiotomy in sensitivity analyses, these associations were no longer statistically significant. These findings suggest an association between perineal morphometry and spontaneous perineal tears, but do not support using perineal measurements as standalone markers.