Objectives To evaluate the changing association between lateral episiotomy and obstetric anal sphincter injury (OASIS) for women with low and high baseline risk of OASIS.
Design A population-based register study.
Setting Data gathered from the Finnish Medical Birth Register for the years 2004−2011.
Participants All women with spontaneous vaginal or vacuum-assisted singleton births in Finland (n=384 638).
Main outcome measure OASIS incidence.
Results During the study period, the incidence of OASIS increased from 1.3% to 1.7% in women with first vaginal births, including women admitted for first vaginal birth after a prior caesarean section and from 0.1% to 0.3% in women with at least one prior birth, whereas episiotomy rates declined from 56.7% to 45.5% and 10.1– 5.3%, respectively. At the study onset, when episiotomy was used more widely, it was negatively associated with OASIS in women with first vaginal births, but as episiotomy use declined it became positively associated with OASIS. Women with episiotomy were complicated by OASIS with clearly higher risk scores than women without episiotomy suggesting that episiotomy was clearly protective against OASIS. OASIS occurred with lower mean risk scores among women with and without episiotomy over time. However, OASIS incidences increased only among women with episiotomy, whereas it decreased or remained among women without episiotomy.
Conclusions The cross-over effect between episiotomy and OASIS could be explained by increasing disparity in baseline OASIS risk between treated and untreated women, since episiotomy use declined most in women at low OASIS risk. Episiotomy rate can be safely reduced in low-risk women but interestingly along with the policy change the practice to cut the episiotomy became less protective among high-risk women.
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To re-evaluate the association between obstetric anal sphincter injury (OASIS) and lateral episiotomy, for women at varying baseline risk of OASIS based on number of prior vaginal births, mode of delivery (spontaneous vaginal or vacuum assisted) and birth weight.
To assess whether increasing OASIS incidence was mostly attributable to secular changes in risk factors for OASIS, and whether there was measurable confounding by indication, with an association between use of episiotomy and baseline risk of OASIS.
During 2004–2011 total episiotomy rates declined, while OASIS incidences increased among both women with first vaginal births and women with prior vaginal births.
An increasingly positive association between episiotomy and OASIS was observed, as episiotomy was used increasingly restrictively.
The cross-over effect between episiotomy and OASIS could be explained by increasing disparity in baseline OASIS risk between treated and untreated women, since episiotomy use declined most in women at low OASIS risk.
The observed statistical association between lateral episiotomy and OASIS was profoundly modified by episiotomy rate, serving here as a surrogate for unmeasured confounding by indication.
Strengths and limitations of this study
The strength was that data source was a mandatory national database that covers the entire Finnish population.
We were not able to include all known risk factors for OASIS in the models such as length of the second stage of labour, episiotomy indications and fetal distress.
Obstetric anal sphincter injury (OASIS) is a serious complication of vaginal delivery with up to two-thirds of women affected by it suffering from anal incontinence.1 Although there is an overall increase in the incidence of OASIS, there are substantial differences in incidence between countries, ranging from 0.2% to 4.0% in 2010 in Europe2 and from 3.5% to 5.9% in the USA3 ,4 in the last decade. It is unclear whether such trends reflect differences in populations, in diagnosis and registration, or differences in management of delivery between and within countries.5 ,6 The association between episiotomy and OASIS remains a controversial source of debate. The available evidence from randomised controlled studies suggests that restrictive use of episiotomy leads to less need for suturing, better healing outcomes and reduced severe perineal trauma (RR 0.67, 95% CI 0.49 to 0.91, 7 trials, n=4404).7 However, further, large observational studies have suggested that although the midline type of episiotomy is associated with increased OASIS risk,8 the mediolateral episiotomy technique is associated with substantially decreased OASIS risk.9
Lateral episiotomy, the exclusive episiotomy technique practiced in Finland, is when an incision is made to the vaginal introitus 1 or 2 cm lateral to the midline and directed towards the ischial tuberosity.10 Our previous work demonstrated that lateral episiotomy was associated with a lower OASIS incidence in first births and a higher rate in second and subsequent births,11 but use of episiotomy had declined substantially over the decade to 2007 among both women with first vaginal births and multiparous women as the incidence of OASIS increased.12 As the use of episiotomy has become even more restrictive, the aim of this retrospective case–control study was to re-evaluate the association between OASIS and lateral episiotomy, for women at varying baseline risk of OASIS based on number of prior vaginal births, mode of delivery (spontaneous vaginal or vacuum assisted) and birth weight. We wished to assess whether increasing OASIS incidence was mostly attributable to secular changes in risk factors for OASIS, and whether there was measurable confounding by indication, with an association between use of episiotomy and baseline risk of OASIS.
Materials and methods
Data and population
The data were gathered from the Finnish Medical Birth Register (MBR) that includes information on maternal and neonatal birth characteristics and perinatal outcomes during seven postnatal days for all births after the 22nd gestational week or weighing 500 g or more. The authorisation to utilise a sensitive health register data for the period from 2004 to 2011 was granted by THL National Institute for Health and Welfare, the current register keeper, according to national data protection legislation.
Incidence of OASIS is defined by the International Classification of Diseases (ICD-10) codes O70.2 (3rd degree) and O70.3 (4th degree), and has been captured in the MBR since 2004, as well as the Hospital Discharge Register (HDR). The two data sources were linked together using encrypted unique personal identification numbers. We used only anonymised data, and accordingly informed consent of the registered individuals was not needed. The HDR also provided information on other medical interventions and surgical procedures during pregnancy and birth. Women with a second or subsequent OASIS (n=34) were excluded from analysis.
The study population included all women with singleton vaginal spontaneous or vacuum-assisted delivery (n=384 638) during the study period from 2004 to 2011. The deliveries were categorised into two groups based on number of prior vaginal births; women with first vaginal births including women admitted for first vaginal birth after a prior caesarean section (CS), that is, first vaginal birth (n=168 081) and women with at least one prior vaginal birth (n=216 557). The study period was grouped into four 2-year periods to compute the secular trends in OASIS incidences, episiotomy rates and risk profiles during the study period.
Variables and definitions
The degree of OASIS was classified according to standard definitions; a third-degree injury involves the external anal sphincter and a fourth-degree injury affects both the anal sphincter and the anorectal mucosa.13 Third-degree injuries and fourth-degree injuries were pooled for all the analyses. Mode of delivery was classified either vaginal spontaneous or vacuum–assisted; forceps and breech were excluded from the analytic dataset due to low number of cases. Breech, forceps and CS rates are presented as additional information. Gestational age was estimated based on data for the last menstrual period, unless there was a discrepancy of more than 7 or 14 days at the first-trimester or second-trimester ultrasound measurements, respectively. Smoking during pregnancy was self-reported and grouped as non-smoking, given up smoking during the first trimester and continued smoking after the first trimester.
To assess association of episiotomy and OASIS over time and risk group, we computed risk scores for OASIS based on OASIS incidences for all years pooled for subgroups of women, by number of prior vaginal births, mode of current delivery (vaginal spontaneous or vacuum assisted) and mode of prior delivery (CS or vaginal), separately for four birth weight strata (<3000, 3000–3499, 3500–3999 and ≥4000 g). We calculated risk scores separately for women with first vaginal births, including women admitted for first vaginal birth after a prior caesarean section, and women with at least one prior vaginal birth. For both groups, women with the lowest OASIS incidence were assigned a risk score of 1.0 and those with higher risks had each risk factor multiplied by the relative increase in the OASIS incidences compared to this reference group. Thus, risk scores did not allow comparison between the two groups of women. Factors used in the risk scores (number of prior vaginal births, prior CS before first vaginal delivery, birth weight and mode of delivery) were chosen as recognised independent risk factors for OASIS from previous analyses of this register. We defined two risk strata (low and high) for OASIS during the entire study period separately for both groups of women, to assess possible changes in OASIS risk profiles over time based on receiver operator characteristic (ROC) curves (not shown). The best thresholds to maximise sensitivity and specificity were 1.85 (sensitivity 49% and specificity 72%) in women with first vaginal births and 2.50 (sensitivity 50% and specificity 76%) in women with at least one prior vaginal birth that were chosen as cut offs between low-risk and high-risk strata for both groups. Risk scores ranged from 1.0 to 4.9 in women with first vaginal birth and from 1.0 to 14 in women with at least one prior vaginal birth.
Statistical tests were performed separately for both groups of women with and without OASIS or with and without episiotomy. Statistical differences in categorical variables were evaluated by χ2 test and differences in continuous variables by Mann-Whitney U, Kruskal-Wallis, Student's t or analysis of variance (ANOVA) tests as appropriate. Logistic regression analyses were used to calculate OR of OASIS with 95% CI adjusting for statistically significant independent and clinically important risk factors for OASIS (p<0.1) including maternal age, gestational age, prior CS, birth weight, mode of delivery, episiotomy, epidural analgesia and maternal smoking. Information on length of active second stage of birth was missing in 83 156 (21.7%) cases and thus this variable was not included in analyses. The data were analyzed using Predictive Analytics SoftWare V.19.0.
For the women with first vaginal births, including women admitted for first vaginal birth after a prior caesarean section (n=168 081), with singleton vaginal spontaneous or vacuum-assisted births, episiotomy rate decreased between 2004–2005 and 2010–2011 from 56.7% to 45.5% while OASIS incidences increased from 1.3% in 2004–2005 to 1.7% in 2010−2011 (table 1). For women with at least one prior vaginal birth (n=216 557), episiotomy rate almost halved from 10.1% to 5.3% while OASIS incidences increased from 0.2% in 2004–2009 to 0.3% in 2010–2011. As episiotomy use became more restrictive, OASIS incidences increased. After adjustment episiotomy was positively associated with OASIS among both groups (figure 1). Furthermore, OASIS incidences decreased in women who gave birth without an episiotomy while increased in women with episiotomy among both groups of women (figure 2).
Secular trends of demographics and obstetric interventions were modest (table 1) with the exception of a slight increase in use of vacuum assistance (unadjusted OR (uOR) 1.24, 95% CI 1.20 to 1.29 and uOR 1.28, 95% CI 1.19 to 1.38 in 2010–2011 compared with 2004–2005 in women with first vaginal births and women with prior vaginal births, respectively, data not shown) and epidural analgesia (uOR 1.15, 95% CI 1.12 to 1.18 and uOR 1.18, 95% CI 1.15 to 1.22, respectively in 2010–2011 compared with 2004–2005, data not shown). Furthermore, risk scores (table 2) among those women with first vaginal births (mean 1.60, range 1.00−4.90) were constant across the 8-year period, whereas the women with prior vaginal births showed a slight but statistically significant difference in risk profile across the 2-year time periods (mean 2.23 to 2.28, range 1.00 to 14.00; table 3).
Episiotomy use declined both among women at low and high risk of OASIS in both groups but most among women at low risk (table 4). Use of episiotomy decreased significantly by 27% (uOR 0.58, 95% CI 0.56 to 0.60) and 9% (uOR 0.74, 95% CI 0.70 to 0.79) among women with first vaginal births with low-risk and high-risk scores, respectively. Among women with prior vaginal births, episiotomy use decreased by 52% (uOR 0.45, 95% CI 0.43 to 0.48) and 36% (uOR 0.60, 95% CI 0.56 to 0.65) among women with low-risk and high-risk scores, respectively. Correspondingly, OASIS incidences increased most among women at low risk of OASIS among both groups but also among women with prior vaginal births at high risk of OASIS. The incidence of OASIS increased significantly by 63% (uOR 1.60, 95% CI 1.37 to 1.88) in women with first vaginal delivery at low risk, but not in women with first vaginal births at high risk, and doubled in women with prior vaginal births at both low risk and high risk (uOR 2.24, 95% CI 1.51 to 3.35) and (uOR 1.69, 95% CI 1.17 to 2.45), respectively. Furthermore, in both groups regardless of risk group OASIS incidences increased in women with episiotomy, whereas decreased or remained the same in women without episiotomy.
Table 3 shows the mean risk scores of OASIS, episiotomy and OASIS with and without episiotomy. The mean risk scores of women who underwent episiotomy increased over time and were significantly higher than in women without episiotomy. The mean risk scores of women affected by OASIS decreased during the study period among both groups. Further, women with episiotomy were complicated by OASIS with higher mean risk scores than women without episiotomy in both groups as an indication of a protective effect brought about by episiotomy.
During the study period in 2004–2011 total episiotomy rates declined, while OASIS incidences increased among both women with first vaginal births including women admitted for first vaginal birth after a prior caesarean section and women with at least one prior vaginal birth women. Paradoxically, we observed an increasingly positive association between episiotomy and OASIS, as episiotomy was used increasingly restrictively. Across a wide range of known risks for OASIS, we observed that the OASIS risk profile for the overall population was constant during the years studied, not explaining this increased OASIS incidence. However, episiotomy use declined sharply among women at low baseline risk, while conversely OASIS incidences increased among women with episiotomy while decreasing or remaining the same among women without episiotomy. Thus, the observed statistical association between lateral episiotomy and OASIS was profoundly modified by episiotomy rate, serving here as a surrogate for unmeasured confounding by indication. These results, in parallel with those from randomised trials, suggest that the episiotomy rate can be safely reduced in low-risk women, without any increase in OASIS rate.
Despite the positive associations between episiotomy and OASIS, the likely confounding by indication, our results suggest that episiotomy still had a protective role since women with episiotomy were complicated by OASIS with higher mean risk scores than women without episiotomy as shown in table 3. However, it appeared that mean risk scores among women with OASIS and with episiotomy decreased over time suggesting that along with the policy change in episiotomy use the technique became less protective also among high-risk women. Other data have suggested that narrow-angled episiotomy increases the risk of OASIS and measures such as point of incision, episiotomy length and depth might reduce the OASIS risk.14 However, due to observational study design we were able to reveal only associations between the role of episiotomy and OASIS, and therefore well-designed randomised controlled trials of restrictive versus routine lateral episiotomy among high-risk women are still needed.
The most important strength of this study was that the data covered the total population of women with vaginal births over an 8-year period in Finland. Differences in registration routines might have affected OASIS incidences, but both national registers used for this study have extensively validated data quality.5 ,15 ,16 The possible limitation was that we were not able to include all known risk factors for OASIS in the models, including the prolonged second stage of birth and abnormal presentation. Furthermore, the data also lack information on all possible indications for episiotomy, including fetal distress, slow crowning and condition of the perineum at birth that might have biased our results. The study results are likely generalisable only to lateral episiotomy.
Using an 8-year population-based data we showed that the highest risk of OASIS was associated with high birth weight with vacuum-assisted birth regardless of number of prior vaginal births but the incidences of OASIS were in the order of sevenfold after the first vaginal birth compared to subsequent vaginal births. The observed association between lateral episiotomy and OASIS, was profoundly modified by episiotomy rate, serving here as a surrogate for unmeasured confounding by indication.17 Variation in episiotomy rate must contribute to the heterogeneity in previous estimates of this association, both between observational studies in different countries with different episiotomy policies, and studies within the same countries over time. Randomised trials are required to assess the optimal rate of lateral episiotomy for women at high risk of OASIS.
We thank Tuomas Selander, MSc, for drawing the figures.
Contributors All authors participated in designing the study. SR managed the dataset and performed statistical analyses. RC, MG, MRK, KL, MRJ and SH were statistical advisors. All authors contributed to the interpretation of the results, as well as to writing and editing the manuscript.
Funding This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None.
Ethics approval Permission to use the confidential register data in this study was approved on 10 October 2012 by the National Institute for Health and Welfare (THL) in Finland (Reference number 263/05.05.00/2012).
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement No additional data are available.