The association of maternal obesity and obstetric anal sphincter injuries (OASIS) at time of vaginal delivery

The risk of third and fourth-degree perineal laceration following vaginal delivery in obese patients is relatively understudied and has mixed findings in existing literature. To examine the association of maternal obesity and obstetric anal sphincter injuries following vaginal delivery. The Healthcare Cost and Utilization Project's National Inpatient Sample was retrospectively queried to examine 7,385,341 vaginal deliveries from January 2017 to December 2019. The exposure assignment was obesity status. The main outcomes were third and fourth-degree perineal lacerations following vaginal delivery. A statistical analysis to examine the exposure-outcome association included: (i) inverse probability of treatment weighting (IPTW) with log-Poisson regression generalized linear model to account for pre-pregnant and pregnant confounders for the exposure; and (ii) multinomial regression model to account for delivery factors in the IPTW cohort. The secondary outcomes included (i) the temporal trends of fourth-degree laceration and its associated factors at cohort level and (ii) risk factor patterns for fourth-degree laceration by constructing a classification-tree model. In the IPTW cohort, obese patients were less likely to have fourth-degree lacerations (2.3 vs 3.9 per 1,000, adjusted-odds ratio [aOR] 0.62, 95% confidence interval [CI] 0.56-0.69) and third-degree lacerations (15.6 vs 20.1, aOR 0.79, 95%CI 0.76-0.82) compared to non-obese patients. In contrast, forceps delivery (54.7 vs 3.3 per 1,000, aOR 17.73, 95%CI 16.17-19.44), vacuum-assisted delivery (19.7 vs 2.9, aOR 5.18, 95%CI 4.85-5.53), episiotomy (19.2 vs 2.8, aOR 3.95, 95%CI 3.71-4.20), and shoulder dystocia (18.0 vs 3.4, aOR 2.60, 95%CI 2.29-2.94) were associated with more than two-fold increased risk of fourth-degree perineal laceration. Among the obesity group, patients who had forceps delivery and shoulder dystocia had the highest incidence of fourth-degree laceration (105.3 per 1,000). Among the non-obesity group, patients who had forceps delivery, shoulder dystocia, and macrosomia had the highest incidence of fourth-degree laceration (294.1 per 1,000). The incidence of fourth-degree perineal laceration decreased by 11.9% over time (P-trend=0.004); forceps delivery, vacuum-assisted delivery, and episiotomy also decreased by 3.8%, 7.6%, and 29.5%, respectively (all, P-trend<0.05). This national-level analysis suggests that obese patients are less likely to have obstetric anal sphincter injuries at the time of vaginal delivery. Furthermore, this analysis confirms other known risk factors for fourth-degree laceration such as forceps delivery, vacuum-assisted delivery, episiotomy, and shoulder dystocia. We noted a decreasing trend in fourth-degree lacerations which may be due to evolving obstetric practices.