Spontaneous obstetric anal sphincter injury among nulliparous women with non-operative vaginal delivery: Modifiable risk factors?

Abstract

Obstetric anal sphincter injury has significant maternal morbidity and sequelae. While certain risk factors (e.g. operative vaginal delivery) have been well established, spontaneous obstetric anal sphincter (sOASIS) has been less studied. We aimed to identify risk factors for sOASIS in nulliparous women with a non-operative vaginal delivery. This is a nested case-control study within a large, NIH-funded retrospective cohort study of patients with diabetes, hypertensive disorders, and/or fetal growth restriction (FGR) affecting singleton pregnancies at a tertiary-care hospital from January 2002 to March 2014. Pregnancies with multiparity, FGR, pre-labor or intrapartum cesarean delivery, episiotomy, and operative vaginal delivery were excluded. Women with sOASIS were compared to those without sOASIS. Maternal and neonatal characteristics were compared between groups. Data were collected by trained nurses. Unadjusted and adjusted odds ratios or mean difference were calculated using multivariable logistic regression. Of the 2,123 deliveries, 82 (3.9%) experienced a sOASIS. Pre-pregnancy obesity (28.1% vs 48.2%, aOR [95% CI] 0.41 [0.25, 0.67]) was the only protective risk factor for a sOASIS. Pregestational diabetes (6.1% vs 1.9%, aOR [95% CI] 3.52 [1.31, 9.44]) and higher birth weight (3,397 vs 3,233 grams, beta coefficient [95% CI] 121 [31, 212]) were associated with sOASIS. Macrosomia (4.9% vs 1.7%) was not statistically different, but was more frequent in the sOASIS group. Induction method, labor type, peripartum infections, and hypertensive diseases were similar between groups. Risk of having a sOASIS during a non-operative vaginal delivery is low and may be preventable. Our cohort suggests pre-pregnancy diabetes is the predominant, yet potentially modifiable risk factor associated with sOASIS in nulliparous women which should be studied further.