Risk factors for obstetric anal sphincter injuries at vaginal birth after caesarean: a retrospective cohort study

More than 85% of women sustain some form of perineal trauma during vaginal childbirth in the UK [1]. Obstetric anal sphincter injuries (OASIS), i.e. third- and fourth-degree perineal tears, involve the anal sphincter resulting in significant morbidity in women and are a contributory factor to longer-term anal incontinence and faecal urgency [2]. The rates of OASIS have increased in the UK from 1.8% in 2000 to 5.9% in 2011, and a recent multicentre survey revealed a median national OASIS rate of 2.85% (0–8%) [3, 4]. Risk factors associated with the development of OASIS are nulliparity, infant macrosomia (> 4 kg), induction of labour, prolonged second stage of labour, maternal age and operative vaginal delivery (OVD) [5].

There is a consensus that women who delivered previously with single, uncomplicated caesarean section (CS), and with otherwise uncomplicated current pregnancy, should be encouraged to attempt a vaginal delivery [6, 7].

Although success rates of planned vaginal birth after caesarean section (VBAC) have been quoted to be 63.4–75%, there has been a reported overall decline in attempts to labour after a previous CS [7, 8, 9]. This, accompanied by rising rates of primary caesarean, has been a significant driver for the increased CS rate [9].

Research has shown an association between VBAC and an increased risk of OASIS compared with both primparous (adjusted OR 1.42, 95% CI 1.25–1.6. p < 0.001) and multiparous (OR 13.6; 95% CI, 4.7–39.3; p < 0.001) women [10, 11]. VBAC delivery is also associated with an increased rate of OVD compared with primiparous delivery (39% vs. 30%, OR 1.15, 95% CI 1.01–1.3, p < 0.0001) [9]. This may in part be due to the accoucher’s awareness of the risks that prolongation of the second stage of labour have on the risk of uterine scar rupture. Furthermore, OVD further potentiates the risk of OASIS at VBAC, especially with the use of forceps (58% OASIS with forceps at VBAC versus 33% vacuum, p = 0.001) [9]. It was also speculated that the increased rate of complicated delivery and OASIS was due to the relative cephalopelvic disproportion as the indication for the initial CS [12]. It has been suggested that risk factors which led to the initial caesarean are carried over to subsequent delivery [13]. Additionally, these are possibly intensified because of more propulsive, secundiparous contractions coupled with a ‘nulliparous’ perineum [12, 14].

Previous studies relied on administrative coding data, which are known to be prone to error, rather than extracting information directly from individual case records [3]. Additionally, previous work has not studied the influence of baseline characteristics or first labour events on the risk of OASIS at subsequent delivery [9]. The objective of this research is therefore to evaluate whether specific baseline characteristics and urgency of caesarean at first delivery affect subsequent vaginal birthing outcomes, especially regarding sustaining OASIS.

This is a retrospective cohort study carried out through retrospective analysis of prospectively collected data from the University of Southampton NHS Foundation Trusts’ maternity database. Only anonymised data were used, so informed consent was not required. Data included all secundiparous women with previous CS who achieved a singleton, term, cephalic vaginal delivery at the Trust from January 2004 to December 2014.

Data were extracted electronically from a maternity database to include all women within the study period having delivered vaginally after one previous CS. Women who were delivered by repeat CS, breech delivery or who delivered pre-term were excluded from analysis. Data were then extracted from each of the individual electronic maternity records of those meeting the inclusion criteria. This method of data extraction adjusted for any potential data collection inaccuracies.

The data were analysed using IBM SPSS v.22 (IBM Corp., 2016). The Kolmogorov-Smirnov test was used to determine the distribution of continuous data. Data are presented as mean (standard deviation) or median (range) as appropriate for normal and non-normal distributions, or as number (%). Normally distributed data were compared using independent samples t-test and non-normally distributed data by the Mann-Whitney U test. Categorical data were analysed using the chi-square test. For comparisons between continuous and categorical data, parametrically distributed data were analysed using the one-way ANOVA and non-parametric data by the Kruskall-Wallis test. Statistical significance was defined as p < 0.05. In all analyses, data on third- and fourth-degree OASIS were combined.

Permission to undertake the research was granted by our sponsor, University Hospital Southampton NHS Foundation Trust, under registration no. RHM O&G0234. The study was granted full ethical approval by the Health Research Authority North West-Preston Research Ethics Committee; reference no. 15/NW/0782. Patient consent was not required as this was a retrospective data analysis with no direct patient contact.

In the 11-year time period there were 2736 successful, singleton VBAC deliveries. The approximated VBAC success rate for the study period was 70.0%. After excluding all those who did not fit the inclusion criteria (see Fig. 1), the study population was 1375. The prevalence of OASIS was 8.1% (112/1375), of which the vast majority had either 3a or 3b tears (41.1% and 45.5%, respectively) (see Table 1).

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The hospital’s contemporaneous birthing outcomes revealed an overall OASIS rate of 3.1%, including a multiparous OASIS rate of 1.2% and a primiparous OASIS rate of 5.8%. Compared with the primiparous OASIS rate, secundiparous women at VBAC were at a 1.4-fold greater risk [8.1% vs. 5.8%, difference 2.4% (difference 2.4%, 95% CI 1.1, 3.6)]. Secundiparous women were at significantly greater risk than other multiparous women with prior vaginal deliveries (difference 7.0, 95% CI 6.4, 7.6), representing a 6.8-fold increase.

Those with OASIS had significantly heavier babies, with a significantly greater proportion weighing > 4 kg.

Operative vaginal delivery (OVD) at VBAC was 2.4-fold more likely than the Trust’s approximated OVD rate (45.0% vs. 18.9%). While there was no difference in whether the VBAC deliveries were operative, the women sustaining OASIS at VBAC had significantly more forceps deliveries.

Women without OASIS were significantly more likely to have had an MLE. This difference was seen regardless of the type of delivery.

There were no significant differences when analysing the foetal head position [whether occiput posterior (OP) or not], length of second stage of labour, use of regional anaesthesia during second stage or whether the VBAC was post-term or induced. Although the difference was not significant, those sustaining OASIS were 2.5-fold more likely to have delivered a baby with shoulder dystocia than those without OASIS.

When comparing those that sustained OASIS at VBAC with those that did not, there was no difference in whether the initial CS was post-term or whether the CS followed an induction of labour, or in overall cervical dilation at the time of CS decision or whether fully dilated at CS decision (see Table 6).

Of all VBAC deliveries, 78.0% (1071/1375) had an initial CS in labour. Those sustaining OASIS at VBAC were more likely to have been in labour at the initial CS. The OASIS rate of those that had an initial CS whilst in labour was 8.5% (91/1071) compared with 6.9% (21/304) of those that were not in labour at initial CS (p = 0.371).

Although there was no difference if at initial CS there was a non-cephalic presentation, those presenting this way were 1.3-fold less likely to have OASIS at VBAC.

Those that had OASIS at VBAC had significantly heavier babies at the initial CS, but no difference was seen in the proportion of those that had a birth weight > 4 kg.

There was a significant difference when comparing the overall categories of CS; moreover, those sustaining OASIS at VBAC were 1.5-fold more likely to have an urgent CS.

This study has shown that secudiparous VBAC delivery is associated with a significantly increased risk of OASIS, especially if the initial CS was urgent. Additional risk factors were increased maternal age and increasing birthweight. The current patient pathway for VBAC delivery makes no reference to these risks, so our findings provide new data to improve counselling of potential VBAC patients, provision of information and candidate selection [7]. Currently, the only basis of whether a VBAC delivery is “successful” is if the infant is born vaginally. More consideration needs to be made about the potential impact of VBAC delivery on the perineum and the resultant effects this may have on long-term physical, social and psychological wellbeing of patients.