Background. The incidence of anal sphincter tears is highest among nulliparous women. The aim of this study was to ascertain if there were other factors that increased their risk.
Methods. This was a retrospective study of all primigravid vaginal deliveries that had sustained an anal sphincter tear (n = 122), compared with deliveries that did not have this complication (n = 16 050). The study sample was drawn from a computerized maternity information database, comprising 52 916 deliveries in the South Glamorgan region during 1990–99. SPSS version 10 was used for statistical analysis.
Results. The incidence of anal sphincter tears in this study population was 0.8% (122/16172). Postdates (OR = 1.8, 95% CI = 1.3–2.6) and fetal macrosomia (OR = 3.8, 2.4–6) together with induction of labor (OR = 1.5, 1.01–2.2), use of spinal analgesia at delivery (OR = 3.1, 1.1–8.4), assisted vaginal delivery (OR = 1.9, 1.3–2.7; especially the use of forceps, OR = 2.2, 1.3–3.9) and doctor-conducted deliveries (OR = 2.2, 1.6–3.2) were found to be associated with a significantly higher incidence of anal sphincter tears. Logistic regression revealed fetal macrosomia and doctor-conducted deliveries to be independent risk factors that, when occurring together, were associated with a fourfold increase in the risk of occurrence of anal sphincter tears.
Conclusions. This study suggests that careful assessment and counseling of women, particularly > 40 weeks gestation or those potentially having macrosomic fetuses, especially if forceps are to be used for prolonged second stage in primigravid women, may help to identify those at significant risk of anal sphincter tears.