Maternal Ethnicity and the Risk of Obstetrical Anal Sphincter Injury: A Retrospective Cohort Study.

OBJECTIVE

To explore the role of maternal ethnicity as a risk factor for obstetrical anal sphincter injury (OASI).

METHODS

A retrospective cohort study of all women with singleton gestations who had a vaginal delivery at term, between January 2014 and October 2017, at a single center. OASI was defined as a third-degree perineal tear (anal sphincter complex) or a fourth-degree perineal tear (anorectal mucosa). The characteristics of women with and without OASIs were compared. Multiple logistic regression was performed to account for potential confounders, including ethnicity.

RESULTS

During the study period, 11 012 women were eligible for inclusion, of whom 336 (3.1%) had an OASI; 313 (93.1%) had a third-degree tear, and 23 (6.9%) had a fourth-degree tear. Women with OASIs were characterized by younger maternal age (<35 years), Asian ethnicity, nulliparity, neonatal birth weight ≥3500 grams, midline and mediolateral episiotomy, second stage of labour lasting ≥60 minutes, and assisted vaginal delivery. After adjusting for potential confounders, Asian ethnicity remained independently associated with increased risk of OASI (adjusted odds ratio 2.07; 95% CI 1.6-2.7) whereas mediolateral episiotomy was independently associated with decreased risk of OASI (adjusted odds ratio 0.64; 95% CI 0.5-0.9).

CONCLUSION

Asian ethnicity is independently associated with increased risk of OASI. Although midline episiotomy increases the risk of OASI, mediolateral episiotomy may protect against OASI, and should be considered in high-risk patients.