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The rate of obstetric anal sphincter injuries in Finnish obstetric units as a patient safety indicator

European Journal of Obstetrics & Gynecology and Reproductive Biology, 1, 169, pages 33 - 38



To study whether there are significant differences in the rate of obstetric anal sphincter injuries (OASIS) between the different sized delivery units in Finland.

Study design

The study was performed as a population based registry study in Finland, including all births (294 725) between 2006 and 2010. All the Finnish delivery units (34) were categorized by the number of annual deliveries and the OASIS rate was then compared between the different sized delivery units using a logistic regression analysis adjusting for maternal age and parity. The Robson ten group classification was used for more accurate comparison.


The OASIS rate was significantly elevated, both in the largest units with 5000 annual deliveries or more (OR 1.46, 95% CI 1.11–1.92) and in the smallest units with less than 500 annual deliveries (OR 1.33, 95% CI 1.22–1.45). In the Robson's group 1 (primiparous, single cephalic term pregnancy, spontaneous labour) the risk for OASIS was the highest in the largest units (OR 1.44, 95% CI 1.28–1.61) while in the Robson's group 3 (multiparous, single cephalic term pregnancy, spontaneous labour) the highest risk was found in the smallest units (OR 2.90, 95% CI 1.68–5.02).


There is significant inter-hospital variation in OASIS rates suggesting significant differences in obstetric practices. Robson's ten group classification should be used to enhance the inter-hospital comparison.

Keywords: Obstetric trauma, Obstetric anal sphincter injuries (OASIS), Patient safety, Patient safety indicator, Robson ten group classification.

1. Introduction

Over the past two decades patient safety has become one of the most prominent issues in health policy. To provide accurate and reliable data we need patient safety indicators that are easily measurable, comparable and preferably under mandatory reporting systems. Patient safety indicators need to be easy to gather without violating patient confidentiality and privacy. Every second year the Organization for Economic Co-operation and Development (OECD) publishes a new edition of the “Health at Glance” report [1] , which includes a number of general patient safety indicators with comparable data within the OECD countries. In the field of obstetrics the selected indicators were obstetric trauma (3rd or 4th degree anal sphincter injury in vaginal delivery with or without instrument) and birth trauma (injury to neonate).

Obstetric anal sphincter injuries (OASIS) are serious complications of vaginal delivery and may have significant and long-term consequences for women's health later in life [2] and [3]. There have been several studies aimed at identifying risk factors for OASIS and it has been shown that in addition to maternal and neonatal variables many obstetric practices have an impact on the OASIS rate [4], [5], [6], [7], [8], [9], and [10]. Hence, it has been suggested that by employing appropriate labour management and care standards the OASIS rate can be reduced [10], [11], and [12] and therefore it would be a suitable indicator for patient safety.

In Finland the OASIS rate is below the Nordic average, but the rate has been on a steady rise during the last decade, reaching the average of 1.0% in 2010 with a great variation between the delivery units. At the same time, there have been significant changes in obstetric interventions: the use of epidural analgesia (39.0% in 2000 and 44.9% in 2010) and vacuum assistance (6.1% in 2000 and 8.6% in 2010) has increased while the use of episiotomy has decreased (41.8% in 2000 and 24.1% in 2010) [13] and [14].

The aim of this study was to analyse whether there are significant differences in the OASIS rate between different sized delivery units in Finland. We focused on the low-risk population and studied if Robson's “ten groups” classification is useful in the inter-hospital comparison.

2. Materials and methods

Obstetric anal sphincter injury (OASIS) encompasses both 3rd and 4th degree sphincter rupture. A 3rd degree rupture is defined as a partial or complete disruption of the anal sphincter muscles, which may involve either or both the external (EAS) and internal (IAS) anal sphincter muscles. A 4th degree rupture is defined as a disruption of the anal sphincter muscles with a breach of the rectal mucosa. The 3rd degree rupture is further subclassified to grade III a, b or c depending on the severity of the trauma [15] and [16]. These subclasses, however, are pooled together in the International Statistical Classification of Diseases, 10th Revision codes (ICD-10) and the codes used are O70.2 (3rd degree) and O70.3 (4th degree) [17] . In Finland, even with less significant perineal trauma, a standard diagnostic protocol is to perform a rectal examination prior to suturing in order to diagnose a possible anal sphincter injury.

This study was performed as a population-based registry study with 5-year data. All the Finnish delivery units and deliveries in Finland between the years 2006 and 2010 were included in the study with a total population of 294 725 women of which 246 504 had a vaginal singleton birth in hospital. The source of data was the Medical Birth Register (MBR), a mandatory register with high quality and completeness [18] . The MBR includes information on maternal and neonatal birth characteristics and perinatal outcomes (all live born or stillborn infants born after the 22nd gestational week or weighing 500 g or more). Since 2004 the information on suture of a third or fourth degree vaginal lacerations has been collected as check-box data (yes/no) in the register. The validity of the item is shown to be good: 95% of cases reported in the MBR were found in the Hospital Discharge Register [19] .

We categorized all 34 Finnish obstetric hospitals by the number of total annual deliveries into six groups to analyse the OASIS rate ( Table 1 ). The three largest units (5000 annual deliveries or more) formed Group I, comprising the largest proportion of deliveries (27%) in the country. Two of these were university clinics (5291 and 5075 annual deliveries) while the rest of the university clinics fell into the following hospital groups – two into Group II (4249 and 4007 annual deliveries) and one (2436 annual deliveries) into Group III. In Finland, only the five university hospitals are considered as tertiary care clinics, accounting for 36% (105 293) of the deliveries in the country. Group VI consisted of the 5 smallest delivery units (less than 500 annual deliveries).

Table 1 Mode of delivery, obstetric trauma rate and obstetric population by the hospital size (non-adjusted ORs).

Hospital group/number of units Annual deliveries Number of deliveries during study period (share of total) Caesarean section rate Instrumental delivery rate Episiotomy rate * Epid and/or spin analgesia OASIS rate
      Total   Elective         Total Instrumental Non-instrumental
I/3 >5000 80 566 (27%) 18.7% OR 1.22, 95% CI 1.20–1.24 6.9% 9.6% OR 1.21, 95% CI 1.18–1.24 25.4% 57.6% 1.30% OR 1.60, 95% CI 1.46–1.74 4.0% OR 1.26, 95% CI 1.10–1.45 0.94% OR 1.64, 95% CI 1.48–1.83
II/4 3000–4999 73 618 (25%) 15.6% Reference group OR 1 6.5% 8.0% Reference group OR 1 21.8% 55.1% 0.88% Reference group OR 1 3.3% Reference group OR 1 0.64% Reference group OR 1
III/4 2000–2999 43 161 (15%) 15.2% 5.9% 7.5% 26.5% 58.7% 0.77% 3.0% 0.55%
IV/8 1000–1999 56 015 (19%) 15.9% 6.6% 8.6% 21.3% 55.8% 0.82% 3.5% 0.51%
V/10 500–999 34 887 (12%) 14.6% 5.9% 7.5% 16.1% 55.7% 0.74% 2.6% 0.56%
VI/5 <500 6478 (2.2%) 20.8% 1.35, 95% CI 1.29–1.42 9.3% 7.6% OR 0.95, 95% CI 0.87–1.04 15.5% 43.9% 1.07% OR 1.31, 95% CI 1.00–1.72 3.9% OR 1.23, 95% CI 0.28–1.92 0.78% OR 1.35, 95% CI 0.97–1.89
Parturients by the Robson's classification
Robson 1 Robson 2a Robson 3 Robson 4a Robson 5
31.2% (25 112) OR 1.08, 95% CI 1.07–1.10 9.0% (7231) OR 1.46, 95% CI 1.42–1.50 31.4% (25 331) OR 0.83, 95% CI 0.82–0.84 6.5% (5209) OR 0.86, 95% CI 0.83–0.89 9.0% (7 279) OR 0.93 95% CI 0.91–0.95
29.5% (21 688) Reference group OR 1 5.4% (4002) Reference group OR 1 37.8% (27 805) Reference group OR 1 6.7% (4940) Reference group OR 1 9.3% (6827) Reference group OR 1
28.0% (12 099) 6.8% (2927) 36.7% (15 826) 7.7% (3343) 10.2% (4423)
28.9% (16 210) 6.4% (3593) 37.5% (20 979) 7.6% (4276) 10.0% (5574)
28.0% (9755) 6.4% (2234) 39.5% (13 772) 8.8% (3060) 9.6% (3356)
25.7% (1668) OR 0.89, 95% CI 0.86–0.93 5.9% (381) OR 0.96, 95% CI 0.87–1.06 37.7% (2439) OR 1.03, 95% CI 1.00–1.06 8.2% (530) OR 1.09, 95% CI 1.00–1.18 12.1% (784) OR 1.25, 95% CI 1.16–1.33

* P < 0.001 with χ2-test.

Comparisons between the different sized hospitals were performed using a logistic regression analysis adjusting for maternal age and parity or for maternal age. In all of the analyses, OASIS rates were reported as proportion of vaginal deliveries and data on OASIS rates were pooled. The total OASIS rates and the OASIS rates in the non-instrumental (221 380) and instrumental (24 774) vaginal deliveries, both vacuum and forceps-assisted, were analysed separately for each hospital size-group. The number of forceps-assisted deliveries was, however, very low (136). The OASIS rates were further calculated separately for deliveries with a low or high birth weight using 4000 g as the limit. The statistical data were managed with SPSS for Windows 17.0 (SPSS Inc., Chicago, IL).

The Robson ten group classification ( Table 2 ) [20] was added into the analysis to compare similar parturient groups. The system is strictly based on the obstetric characteristics of the population, with mutually exclusive and totally inclusive categories, and the limited information needed is very carefully defined, easy to gather and most frequently very reliably reported. Originally, the classification was designed to make the national as well as international comparisons of obstetric practices more reliable. It has been is shown that woman-based classification systems, especially Robson's, are superior to other classification systems, e.g. urgency or indication based [21] . In this study we focused on the low risk population and hence analysed Robson's groups 1–5.

Table 2 The Robson's ten groups classification [20] .

1 Primiparous women, single cephalic term pregnancy, spontaneous labour
2a Primiparous women, single cephalic term pregnancy, induced labour
2b Primiparous women, single cephalic term pregnancy, elective caesarean section
3 Multiparous women, no previous uterine scar, single cephalic term pregnancy, spontaneous labour
4a Multiparous women, no previous uterine scar, single cephalic term pregnancy, induced labour
4b Multiparous women, no previous uterine scar, single cephalic term pregnancy, elective Caesarean section
5 All multiparous women, with at least one previous uterine scar, single cephalic term pregnancy
6 All primiparous women with a single breech pregnancy
7 All multiparous women with a single breech pregnancy including, women with previous uterine scars
8 All women with multiple pregnancies, including women with previous uterine scars
9 All women with a single pregnancy with a transverse or oblique lie, including women with previous uterine scars
10 All women with single cephalic preterm pregnancy, including women with previous scars

3. Results

The main finding was the elevated risk of OASIS in the largest (OR 1.46, 95% CI 1.11–1.92) and in the smallest (OR 1.33, 95% CI 1.22–1.45, both adjusted by maternal age and parity) delivery units ( Fig. 1 ). The OASIS rates in the mid-sized units showed no differences, and they were merged in one group and used as a reference group. In the largest units the risk of OASIS was elevated both in instrumental (OR 1.26, 95% CI 1.10–1.45) and in non-instrumental vaginal deliveries (OR 1.64, 95% CI 1.48–1.83, Table 1 ). In these sub-classes the risk of OASIS in the smallest units was not different from the reference group.


Fig. 1 OASIS rate according to hospital size versus hospital size, where x-axis is hospital size – number of annual deliveries and y-axis is OASIS rate according to hospital size (the number of annual deliveries).

We found differences both in obstetric practices and in obstetric population between the different sized delivery units ( Table 1 ). The caesarean section rate was elevated in the smallest units (OR 1.35, 95% CI 1.29–1.42). Both the caesarean section rate and the instrumental delivery rate were elevated in the largest units (OR 1.22, 95% CI 1.20–1.24 and OR 1.21, 95% CI 1.18–1.24, respectively). The episiotomy rate was increased in the largest units (OR 1.17, 95% CI 1.15–1.19) and decreased in the smallest units (OR 0.71, 95% CI 0.67–0.76) compared to the mid-sized units.

The proportion of Robson's groups 1 and 2a (primiparous, spontaneous and induced labour) was the highest in the largest obstetric units and the lowest in the smallest units and vice versa for Robson's groups 3 and 4a (multiparous, spontaneous and induced labour). Nearly two-thirds of all parturients fell into the low-risk Robson's groups 1 and 3, and those two groups were therefore of major interest, as they should appear more alike in every unit. Approximately 14% of the deliveries were induced (Robson's groups 2a and 4a). The proportion of Robson's group 5 (women with a previous caesarean section) was highest among the smallest hospitals (OR 1.25, 95% CI 1.16–1.33) and lowest in the largest hospitals (OR 0.93 95% CI 0.91–0.95).

There were differences in OASIS rates between the different Robson's groups ( Table 3 ). As expected, the risk of OASIS was significantly higher in primiparous than in multiparous women. This risk was observed for spontaneous deliveries (Robson's groups 1 vs. 3, OR 6.28 95% CI 5.53–7.13) as well as for induced deliveries (Robson's groups 2a vs. 4a, OR 5.77 95% CI 4.44–7.51). Our study did not show, however, significant differences in the risk of OASIS in a spontaneous labour compared to an induced labour within primiparous (Robson's groups 1 vs. 2a) or multiparous women (Robson's groups and 3 vs. 4a).

Table 3 Mode of delivery and the OASIS rate according to the hospital size by the Robson's group.

Hospital size Robson's group 1 Robson's group 2a Robson's group 3
  Mode of delivery OASIS rate   Mode of delivery OASIS rate   Mode of delivery OASIS rate  
  1 2 4 5     1 2 4 5     1 2 4 5    
>5000 70.1% 17.1% 11.3% 1.5% 2.3% OR 1.44 95% CI 1.28–1.61a 55.5% 19.5% 22.6% 2.4% 2.4% OR 1.34 95% CI 1.06–1.63a 94.9% 2.7% 2.1% 0.4% 0.3% OR 1.37 95% CI 1.06–1.77a
3000–4999 73.3% 16.0% 8.9% 1.7% 1.6%   61.4% 18.0% 18.3% 2.3% 2.0%   95.7% 2.2% 1.6% 0.5% 0.2%  
2000–2999 75.1% 14.7% 8.5% 1.8% 1.5% 62.0% 18.6% 16.8% 2.6% 1.5% 95.8% 2.2% 1.5% 0.5% 0.2%
1000–1999 71.5% 17.3% 9.9% 1.3% 1.5% 56.7% 19.8% 21.7% 1.8% 1.5% 95.3% 2.4% 1.9% 0.4% 0.2%
500–999 73.8% 14.9% 9.9% 1.5% 1.4% 61.7% 17.0% 19.2% 2.1% 1.4% 95.4% 2.4% 1.8% 0.4% 0.2%
<500 69.1% 16.4% 13.4% 1.1% 1.9% OR 1.26 95% CI 0.86–1.86 51.2% 15.7% 28.9% 4.2% 0.4% OR 0.23 95% CI 0.03–1.63a 93.8% 3.2% 2.5% 0.5% 0.6% OR 2.90 95% CI 1.68–5.02a
Robson's group 4a Robson's group 5
Mode of delivery OASIS rate   Mode of delivery OASIS rate  
1 2 4 5     1 2 3 4 5    
91.8% 4.0% 3.4% 0.8% 0.3% OR 0.97 95% CI 0.54–1.74 a 44.4% 9.2% 28.0% 16.8% 1.6% 2.5% OR 1.47 95% CI 1.14–1.90 a
93.0% 3.3% 2.9% 0.8% 0.2%   52.2% 7.5% 26.6% 12.2% 1.4% 1.6%  
93.2% 3.4% 2.6% 0.8% 0.4% 55.6% 7.1% 22.5% 13.0% 1.7% 0.9%
92.4% 3.8% 3.1% 0.7% 0.2% 47.0% 7.0% 30.2% 14.6% 1.2% 1.6%
92.5% 3.5% 3.4% 0.7% 0.4% 51.1% 6.8% 26.8% 14.1% 1.2% 1.3%
91.5% 3.0% 3.8% 1.7% 0.4% OR 1.32 95% CI 0.32–5.46 a 42.6% 5.5% 35.8% 14.8% 1.3% 2.7% OR 2.2 95% CI 1.14–4.22 a

a ORs age-adjusted.

1, non-instrumental vaginal; 2, instrumental vaginal; 3, elective caesarean section; 4, urgent caesarean section; 5, emergency caesarean section.

We analysed the OASIS rates for Robson's groups 1–5 in the different hospital size categories. As shown in Table 3 , in the largest units the risk of OASIS was elevated for Robson's groups 1, 2a (primiparous, spontaneous and induced) and 5 (women with a previous caesarean section). In the smallest units the risk of OASIS was elevated for Robson's groups 3 (multiparous, spontaneous) and 5, and these Robson's groups had the highest OASIS rates. A high caesarean section rate did not protect from OASIS.

The smallest units had the highest rate of deliveries of a newborn weighing 4000 g or more (19%, OR 1.12, 95% CI 1.05–1.18) while in the largest units the rate of these deliveries was the lowest (16%, OR 0.94, 95% CI 0.92–0.96). In these deliveries, however, the largest units had the highest risk of OASIS (OR 1.78, 95% CI 1.50–2.10) while in the smallest units the risk of OASIS was not different from the reference group.


In Finland the total OASIS rate has increased and obstetric practices have changed markedly during the last decade. Obstetric practices have been reported to contribute strongly to the risk of OASIS [1] and [22]. The present study showed that the risk of OASIS also depended on hospital size. The trend in Finland has been towards fewer but larger delivery units. Hemminki et al. questioned this, reporting no significant benefits from centralizing births, if high-risk pregnancies are successfully referred to higher level units [23] . In their study, however, the OASIS rate was not used as a patient safety indicator.

In our study the three largest hospitals had the highest total OASIS rate and high OASIS rates in several subcategories. This might be partly explained by the centralization of high-risk pregnancies, e.g. women with a previous complication during pregnancy or women with a major medical disease like diabetes. It is noteworthy that the risk of OASIS was also elevated in the smallest delivery units and especially for multiparous women (Robson's groups 3 and 5). In these units, the risk of OASIS was increased nearly three-fold for Robson's group 3 and more than double for Robson's group 5.

The fact that the inter-hospital differences in OASIS rates remained even when adjusted for the Robson's classification, highlights the significance of the differences in obstetric practices and in general treatment culture. Similarly, Räisänen et al. have reported up to three-fold differences in OASIS rates among the university hospitals in Finland [19] , also suggesting differences in treatment policies rather than in obstetric population.

Previously, it has been reported that episiotomy and epidural analgesia may increase the risk of OASIS in multiparous women, whereas along with young maternal age (<20 years) the same factors seem to protect primiparous women [9] and [10]. Additionally, the risk of OASIS is increased in the first vaginal delivery [1], [7], [8], and [9], in an induced or instrumental delivery [1], [4], [7], [8], [9], and [10] and in a delivery of a newborn weighing more than 4000 g [1], [4], and [10]. We showed that there are significant inter-hospital differences in the use of the obstetric practices, many of which are known to have an impact on risk for OASIS as explained above. The rates of vacuum assistance and episiotomy were the highest in the largest units and especially the episiotomy rate declined strongly by the hospital size. There might have been confounding by indication, as episiotomy was possibly performed prophylactically more often in those who were at high risk for OASIS. We found remarkable inter-hospital differences in caesarean section rates but the high caesarean section rate in the smallest units did not seem to protect from OASIS. In our study, there were no significant differences in OASIS rates between spontaneous and induced vaginal deliveries.

The strength of our study was that the data covered the entire population and therefore offered a comprehensive picture of OASIS. The data were derived from the mandatory, national, population-based MBR, which has excellent coverage and good data quality [18] . It is possible, however, that this kind of routinely collected register information may include errors and there may be differences in diagnostic registration of cases.

The Robson's ten group classification, based on woman's obstetric history, was useful in the inter-hospital comparison. The classification should be used especially in international comparisons and in health care systems with strong centralization of high-risk parturients. The main bias is that the classification does not take into account woman's possible pre-existing medical condition, which might be a confounding factor when comparing tertiary clinics to low-risk units.

Even though high-risk pregnancies and deliveries are already centralized in Finland, there are significant inter-hospital differences in OASIS rates. In order to minimize the number of OASIS, risk factors such as primiparity and a previous caesarean section – especially without previous vaginal deliveries – should receive special attention and these factors should be considered when choosing the delivery unit. An open reporting system and discussion culture together with continuous local and national training of the doctors and midwives would be likely to decrease the inter-hospital differences in the use of obstetric practices and hence potentially increase the quality of maternal care all over the country.

The OASIS rate is a suitable tool to evaluate patient safety within obstetrics and this figure should be analysed in every delivery unit. As shown in our study, there are several confounding factors and therefore the OASIS rates should be interpreted carefully. The validity of the OASIS rate as a patient safety indicator could be improved by combining it with other obstetric indicators, as well as neonatal outcomes. Thus far the only internationally recommended neonatal indicator is birth trauma [1] , a very rough indicator for a modern health care system, and further research is needed in order to implement more apt indicators.


Apart from the authors stated on the title page no other person, agency or institution has participated in the study. None of the authors have any potential conflicts of interest to be disclosed. There was no special funding for the study.


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a Department of Obstetrics and Gynaecology, Helsinki University Hospital, P.O. Box 140, FIN-00029 HUS, Finland

b THL National Institute for Health and Welfare, P.O. Box 30, FI-00271 Helsinki, Finland

c Nordic School of Public Health, P.O. Box 12133, SE-40242 Gothenburg, Sweden

d Group Administration, Hospital District of Helsinki and Uusimaa, P.O. Box 100, FIN-00029 HUS, Finland

e Hjelt Institute, P.O. Box 40, FI-00014 University of Helsinki, Finland

lowast Corresponding author at: c/o Anna-Maija Tapper, Department of Obstetrics and Gynaecology, Helsinki University Hospital, P.O. Box 140, FIN-00029 HUS, Finland. Tel.: +45 60240621.