Overall incidence of anal sphincter injury in vaginal deliveries was significantly reduced by 50%, from 4% (591/14787) in the first time period (2003–2005) to 1.9% (316/16 922) in the second time period (2008–10). The reduction of the incidence of OASIS was of similar magnitude across all studied subgroups defined by risk factors, for both primiparous and multiparous women ().
Incidence of OASIS in different subgroups of women. Data are presented in frequencies (and numbers). p Values from χ2 test
The incidence of OASIS over the study years is displayed in , demonstrating a reduced incidence of OASIS, which in time follows the implementation of the perineum support programme for the staff. also demonstrates a similar reduction of OASIS incidence for the different delivery methods (operative and spontaneous vaginal delivery) between the two study periods: in spontaneous deliveries the OASIS incidence was reduced from 3.1% (409/13 037) to 1.5% (215/14711) and in ventouse from 9.7% (152/1565) to 4.7% (98/2075). Forceps is less used in our department, but a significant OASIS reduction was also observed in forceps deliveries from 16.2% (30/185) to 2.2% (3/136).
Frequency of obstetric anal sphincter injuries (%) for different delivery methods during the study years.
Population characteristics across the study years
Overall changes in population characteristics between the two time periods were small, but the prevalence of older women (>35 years) was significantly higher in the second period (2008–10), and use of ventouse delivery, episiotomy, epidural and induction of labour was more frequent (). Primiparous women comprised 85% of the women with OASIS, but represented only 53.3% of the overall study population.
In a univariate analysis, higher infant birth weight, larger infant head circumference (data not shown), prolonged second stage of labour, instrumental delivery, shoulder dystocia and persistent occiput posterior presentation were significant OASIS risk factors for primiparous women in the first study period (). In the second study period, the same OASIS risk factors remained significant, except for a prolonged second stage of labour ().
Clinical characteristics and obstetric interventions among primiparous women with OASIS and women without OASIS. Data are presented in frequencies (and numbers). p Values from χ2 test
Looking at the various explanatory variables (such as age, maternal body mass index, foetal weight, etc) and analysing time period solely as an explanatory variable for OASIS (due to the perineal protection programme introduced in the second time period), we observed that the first time period emerged as one of the most important ‘risk factors’ with high OR for OASIS in our study. Without adjusting for any other variables, OR for OASIS in the logistic regression analysis for the first study period as compared with the second was 2.10 (95% CI 1.76 to 2.40).
In a multivariate regression analysis (), large infant birth weight, instrumental delivery, prolonged second stage and occiput posterior presentation were significant risk factors for OASIS in the first study period. In the second study period, when the incidence of OASIS was reduced, only instrumental delivery and foetal occiput posterior presentation remained significant risk factors for OASIS.
Risk factors for OASIS in the multivariate regression model (adjusted OR(aOR) and 95% CI)
Frequency of episiotomy use in spontaneous deliveries of primiparous women was reduced from the first time period to the second, and increased in instrumental deliveries (). When adjusted for risk factors in the multivariate analysis, episiotomy appeared as a protective factor for OASIS in both time periods for primiparous women ().
Primiparous women with a previous caesarean section only, and no previous vaginal delivery (n=440), had an increased OASIS risk compared to women with no previous delivery OR=2.2 (95% CI 1.6 to 3.1), both in the first time period (11.5% and 5.9%, respectively, P=0.001) and in the second (6.7% and 2.9%, respectively, P=0.001). Also in this subgroup, the OASIS incidence was reduced with 50% after implementation of the perineal protection programme. When the various study analyses were performed without this small subgroup of vaginal primiparous women with one previous caesarean only, the study conclusions remained unaltered, as expected due to the small number of women in this subgroup.
In a univariate analysis for multiparous women (), instrumental delivery, prolonged second stage of delivery, shoulder dystocia, large infant head circumference (data not shown) and birth weight were significant risk factors for OASIS in both time periods. The risk of OASIS was markedly reduced from the first to the second time period and the time period for the delivery was one of the most important ‘risk factors’; OR for OASIS in the logistic regression analysis for the first time period as compared with the second was 2.31 (95% CI 1.65 to 3.25).
Clinical characteristics and obstetric interventions among multiparous women with OASIS and women without OASIS. Data are presented in frequencies (and numbers). p-Values from χ2 test
In the multivariate regression analysis (), macrosomia and instrumental delivery significantly increased the OASIS risk for multiparous women in the first time period, but not in the second. In the second time period, none of the identified risk factors for OASIS were significant for multiparous women. However, OASIS cases were few (n=53) in this subgroup of women. In the multivariate analysis the effect of episiotomy was non-significant in both time periods (). However, multiparous women with episiotomy were very few in this study and interpretation of the results should be undertaken cautiously ( and ).