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Obstetrics
Modifiable risk factors of obstetric anal sphincter injury in primiparous women: a population–based cohort study

https://doi.org/10.1016/j.ajog.2013.08.043Get rights and content

Objective

To determine modifiable risk factors and incidence of obstetric anal sphincter injury (OASIS) in primiparous women.

Study Design

We performed a population-based retrospective cohort study, using data from the Danish Medical Birth Registry. The population consisted of primiparous women with a vaginal delivery in the time period 2000-2010. Univariable and multivariable logistic regressions were used to determine risk factors of OASIS. Main outcome measures were incidence of OASIS in first vaginal delivery, odds ratios for possible risk factors: age, body mass index, birthweight, head circumference, gestational age, presentation, induction of labor, oxytocin augmentation, epidural, mediolateral episiotomy, vacuum extraction, forceps, shoulder dystocia, and year of delivery.

Results

Of 214,256 primiparous women with a vaginal delivery, 13,907 (6.5%; 95% confidence interval [CI] 6.4–6.6%) experienced an OASIS. The incidence of OASIS increased in the time period (adjusted odds ratio [aOR], 1.02; 95% CI, 1.02–1.03; P < .0001, per year). We found a protective effect of epidural analgesia (aOR, 0.84; 95% CI, 0.81–0.88; P = .0001). Vacuum extraction without episiotomy was a significant risk factor of OASIS (aOR, 2.99; 95% CI, 2.86–3.12; P < .0001), and episiotomy was protective in vacuum-assisted deliveries compared with vacuum-assisted deliveries without episiotomy (aOR, 0.60; 95% CI, 0.56–0.65; P < .0001). Birthweight was found to be an important nonmodifiable risk factor (aOR, 2.76; 95% CI, 2.62–2.90; P < .0001).

Conclusion

Epidural analgesia in itself was protective against OASIS. Vacuum extraction increased the risk of OASIS, although mediolateral episiotomy was protective when applied in deliveries assisted by vacuum extraction.

Section snippets

Materials and Methods

We performed a population-based cohort study where data were retrieved from the Danish Medical Birth Registry (MBR). Primiparous women with a vaginal singleton delivery at term (fully 37 weeks of gestation) in the time period 2000-2010 (n = 214,256) were included. An OASIS was classified according to the Royal College of Obstetricians and Gynaecologists classification.24 A third-degree OASIS was defined as a partial or complete disruption of the anal sphincter muscles, which may involve only

Results

Of 214,256 women with a first vaginal delivery in 2000-2010, 13,907 (6.5%; 95% CI, 6.4–6.6%) had an OASIS. Of these, 11.5% (n = 1599) had a fourth degree OASIS. Table 1 shows distribution of the potential risk factors in women with and without OASIS. Univariable analyses (Table 2) showed that all factors included in the analysis were significant risk factors, except breech presentation, which was a significant protective factor.

We found an interaction between vacuum extraction and mediolateral

Comment

We found that mediolateral episiotomy was protective against OASIS in primiparous women, when used in deliveries assisted by vacuum extraction. We found no protective effect of mediolateral episiotomy when used without vacuum extraction, which is in accordance with some previous findings,8, 10, 13 but in contrast to other findings.5, 21, 23 Vacuum extraction without mediolateral episiotomy was an important risk factor of OASIS, which is in accordance with previous findings.5, 6, 7, 8, 9, 11, 12

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  • Cited by (105)

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      Citation Excerpt :

      In spontaneous vaginal births the rate of episiotomy increased by 43% (16.2%–23.2%). Our study also saw an increase in episiotomy rates; by 14% in instrumental births (56.5%–62.1%) and 40% in spontaneous vaginal births (7.4%–10.4%) though our baseline rates were much lower than those reported by Mohiudin et al.18 Routine episiotomy has not been found to reduce the risk of SPT in a systematic review22 though several observational studies have demonstrated a protective effect in instrumental births.23,24 In our study of nulliparous women, factors increasing the chance of severe perineal trauma included Asian country of birth, forceps birth, neonate ≥4000 g and maternal age.

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    The authors report no conflict of interest.

    Reprints not available from the authors.

    Cite this article as: Jangö H, Langhoff-Roos J, Rosthøj S, et al. Modifiable risk factors of obstetric anal sphincter injury in primiparous women: a population–based cohort study. Am J Obstet Gynecol 2014;210:59.e1-6.

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