(716) Sonographic estimated fetal weight and rates of OASIS among patients undergoing forceps assisted vaginal delivery

Objective: Birthweight is a known risk factor for obstetric anal sphincter injury (OASIS), however, little is known about sonographically determined estimated fetal weight (EFW) and its association with adverse outcomes, such as OASIS, with forceps assisted vaginal deliveries (FAVD). The objective of this study is to evaluate if sonographic EFW ≥ 3500 grams (g) is associated with OASIS, as well as other adverse neonatal and maternal outcomes among FAVD.

Study Design: Retrospective cohort study including patients who underwent FAVD between 2017 and 2021 and who had a sonographic EFW within 28 days of delivery. The primary outcome was the rate of OASIS in patients with EFW < 3500g compared to ≥ 3500g. Secondary outcomes included neonatal composite of sub-galeal, cephalohematoma, humerus or clavicle fracture, facial laceration, and NICU admission (adverse neonatal composite), and quantitative blood loss (QBL). Outcomes were assessed via univariate analyses (Chi-Square test and Mann-Whitney U test) and multivariate logistic regression models and were adjusted for provider clustering and patient characteristics.

Results: 455 records were included, 356 (78.2%) with EFW < 3500 and 99 (21.8%) with EFW ≥ 3500.  Patients with EFW < 3500 were more likely to require FAVD due to non-reassuring fetal heart tracing (p=0.03, Table 1). Remaining demographics were similar between the two groups (Table 1). Patients with EFW ≥ 3500 were not at increased odds of OASIS compared to those with EFW < 3500 (OR 1.39 [0.77-2.52], Table 2). When assessed as a continuous variable, there was no association between increasing sonographic EFW and OASIS rates (OR 1.001 [1.00-1.002)]. Additionally, there was no difference in adverse neonatal composite or QBL between the two groups (Table 2).

Conclusion: Sonographic EFW ≥ 3500g within 28 days of delivery does not appear to be associated with adverse maternal or neonatal outcomes with FAVD. Further research is needed with larger sonographic EFW to further assess the relationship with adverse outcomes with FAVD.