Association between birth weight and head circumference and obstetric anal sphincter injury severity

Objective: To identify risk factors for increasing severity of OASI and evaluate its possible correlation with two known risk factors–birth weight (BW) and neonatal head circumference (HC). Methods: We performed a retrospective cohort study at a university affiliated hospital between 2003 and 2019. We compared five groups of patients – according to presence and degree of perineal laceration – 3a, 3b, 3c and 4 and patients without OASI. Additionally, four parameters were defined to assess their relation to the severity of OASI: (1) BW ≥ 90th and HC < 90th percentiles; (2) BW < 90th and HC ≥ 90th percentile; (3) BW and HC ≥ 90th percentile and (4) BW and HC < 90th percentiles. Results: During the study period, 150,221 deliveries were evaluated. Parturients were allocated according to OASI severity − 455 patients had a 3rd (3a, 3b or 3c) or 4th degree perineal tear, while 110,966 patients had no OASI. Allocation to subgroups according to offspring anthropomorphic measures showed that as fetal size parameters increased, the rate of more severe tears increased, particularly in women delivering neonates with HC ≥ 90th percentile. Multinomial regression analysis was performed for each 3rd degree OASI subgroup. This analysis demonstrated the odds for OASI gradually increase with tear severity for BW ≥ 90th percentile and the combined BW and HC ≥ 90th percentile parameters, as compared with neonates with HC and BW < 90th percentile. Conclusions: Birth weight and head circumference above the 90th percentile are correlated with increased degree of OASI severity.