OASIS in nulliparous and multiparous Asian women: When is episiotomy appropriate?

A retrospective study from the Chinese University of Hong Kong (CUHK) finds episiotomy to be protective against obstetric anal sphincter injury (OASIS) among nulliparous Asian women with singleton normal vaginal or instrumental delivery as well as multiparous women undergoing instrumental delivery, but not multiparous women having normal vaginal delivery.

In Hong Kong, the incidence of OASIS, a serious complication of vaginal delivery that can result in faecal incontinence, increased from 0.04 percent in 2004 to 0.1 percent in 2009, and to 0.3 percent in 2014. In the same period, the use of episiotomy, which is commonly performed during the second stage of labour to facilitate delivery and prevent excessive stretching of perineal muscles, has been in decline in Hong Kong, with rates falling from 81 percent in 2004 to 66.2 percent in 2009, and 47.4 percent in 2014. [Hong Kong College of Obstetricians and Gynaecologists, Territory-wide Audit in Obstetrics & Gynaecology, 2014; Obstet Gynecol Surv 1995;50:821-835]

“The role of episiotomy in mitigating OASIS remains controversial,” wrote the researchers. “Ethnic differences in pelvic floor biometry and pelvic organ mobility have been reported, with studies suggesting that Asian women are more prone to OASIS. This study aimed to review the incidence of OASIS in our unit over the past decade in the context of declining episiotomy rates.” [Int J Gynaecol Obstet 2023;162:632-638; Ultrasound Obstet Gynecol 2017;50:242-246; Aust N Z J Obstet Gynaecol 2018;58:79-85; J Gynecol Obstet Hum Reprod 2019;48:657-662]

The retrospective study included 43,732 singleton normal and instrumental vaginal deliveries, which took place at Prince of Wales Hospital in Hong Kong between 2012 and 2021. When indicated, a left mediolateral episiotomy and a hands-on approach to protect the perineum were used by both midwives and doctors. [Hong Kong Med J 2026;32:6-12]

The episiotomy rate decreased from 62.8 percent in 2012 to 44.7 percent in 2021 (p<0.001), while the OASIS rate increased from 0.3 to 1.4 percent over the same period (p<0.001). “The rising trend of OASIS over the past decade may be attributable to improvements in clinical detection following the promotion of more thorough post-delivery assessments by both midwives and obstetricians,” explained the researchers.

Episiotomy rates among women with and without OASIS were 51.8 and 53.3 percent, respectively. A higher proportion of women in the OASIS group were nulliparous (79.1 vs 51.4 percent; p<0.001) and vaginally nulliparous (85.9 vs 56.5 percent; p<0.001). Instrumental delivery was also more common in the OASIS vs non-OASIS group (29.1 vs 9.3 percent; p<0.001). Women with OASIS had a lower BMI, a longer duration of labour, and delivered heavier neonates.

Among nulliparous women, the OASIS rate was significantly lower with vs without episiotomy vs in both normal vaginal deliveries (0.6 vs 1.7 percent; p<0.001) and instrumental deliveries (1.7 vs 42.9 percent; p<0.001). The rate of OASIS was also significantly lower with episiotomy in instrumental deliveries among multiparous women (0.5 vs 23.5 percent; p<0.001). Overall, episiotomy was a protective factor against OASIS (odds ratio, 0.273; 95 percent confidence interval, 0.208–0.358; p<0.001).

However, in normal vaginal deliveries among multiparous women, the rate of OASIS was significantly lower without rather than with episiotomy (0.3 vs 0.5 percent; p=0.026). “In light of these findings, we recommend a more restrictive approach to episiotomy among multiparous women undergoing normal vaginal delivery,” advised the researchers.