The main objective of this study was to evaluate the association between maternal and fetal anthropometric characteristics and third- and fourth-degree perineal tears. This retrospective cohort study considered all consecutive pregnancies from 2011 to 2017 at a single Institution. The inclusion criteria were: singletons who delivered vaginally during the study period, the presence of information on maternal pre-pregnancy weight, maternal height, and weight of the newborn. The feto-maternal body-mass index (BMI) was calculated as neonatal weight in kg on maternal height in squared meters (kg/m2). In total, 5397 singleton-term pregnancies were included; the prevalence of third-fourth-degree perineal tears was 0.47%. The most predictive factors were: nulliparity, feto-maternal BMI, neonatal weight, gestational age at delivery, and neonatal head circumference. After adjustment in multivariate analysis, the only independent predictors were nulliparity and fetomaternal BMI. The AUC of the final multivariate model was 73.54% (95% CI 65.65–81.42). Furthermore, feto-maternal BMI and gestational age had a significant direct correlation. Nulliparity and feto-maternal BMI are the two best predictors for third and fourth-degree perineal tears in our setting. Confirming this association in future research and integrating it into a decision algorithm on delivery timing could reduce obstetric damage to the anal sphincter.
