Risk factors for obstetrical anal sphincter lacerations.

The objective of this study was to identify the rate of anal sphincter lacerations in a large population-based database and analyze risk factors associated with this condition. Data were obtained from Pennsylvania Healthcare Cost Containment Council (PHC4) regarding all cases of obstetrical third and fourth degree perineal lacerations that occurred during a 2-year period from January 1990 to December 1991. Modifiable risk factors associated with this condition were analyzed, specifically episiotomy, forceps-assisted vaginal delivery, forceps with episiotomy, vacuum-assisted vaginal delivery, and vacuum with episiotomy. There were a total of 168,337 deliveries in 1990 and 165,051 deliveries in 1991 in Pennsylvania. Twenty-two percent (n = 74,881) of the deliveries were by cesarean section and were excluded from analysis. Among the remaining 258,507 deliveries, there were 18,888 (7.3%) third and fourth degree lacerations. Instrumental vaginal delivery, particularly with use of episiotomy, increased the risk of laceration significantly [forceps odds ratio (OR): 3.84, forceps with episiotomy OR: 3.89, vacuum OR: 2.58, vacuum with episiotomy OR: 2.93]. Episiotomy on the whole was associated with a threefold increase in the risk of sphincter tears. However, episiotomy in the absence of instrumental delivery seems to be protective with an OR of 0.9 [95% confidence interval (CI): 0.88-0.93]. Instrumental vaginal delivery, particularly forceps delivery, appears to be an important risk factor for anal sphincter tears. The risk previously attributed to episiotomy is probably due to its association with instrumental vaginal delivery. Forceps delivery is associated with higher occurrence of anal sphincter injury compared to vacuum delivery.