Abstract: The search strategy identified six potentially eligible studies. Two studies were excluded. We included four randomised controlled trials that randomised a total of 599 women (data were available for 595 women). Two trials (283 women) compared the effects of perineal HAase injection during the second stage of labour with placebo injection and were at low risk of bias. Three trials (one three-armed trial was analysed twice) (373 women) compared the effects of perineal HAase injection during second stage of labour with no intervention and two out of the three trials were at high risk of bias. Data from four trials involving 599 women suggested that perineal HAase injection during second stage of labour had a lower incidence of perineal trauma (average risk ratio (RR) 0.69, 95% confidence interval (CI) 0.50 to 0.95,Tau² = 0.08, I² = 82% compared with women in the control group, but there was no clear evidence of a reduction in the incidence of episiotomy (average RR 0.74, 95% CI 0.43 to 1.29, Tau² = 0.17, I² = 66%), first and second degree perineal lacerations (average RR 0.54, 95% CI 0.38 to 1.33, Tau² = 0.30 , I² = 85%) and third and fourth degree perineal lacerations (RR 0.12, 95% CI 0.01 to 2.13). Data from two trials involving 283 women indicated that there was no clear evidence of a reduction in the incidence of perineal trauma (RR 0.90, 95% CI 0.77 to 1.06, Tau²=1.07, I² = 7%), episiotomy (RR 0.77, 95% CI 0.32 to 1.89, Tau² = 0.27, I² = 54%), first and second degree perineal lacerations (RR 1.08, 95% CI 0.83 to 1.40, Tau² = 1.11, I² = 10%) and third and fourth degree perineal lacerations (RR 0.12, 95% CI 0.01 to 2.13) with perineal HAase injection. Data from three trials involving 373 women suggested that perineal HAase injection during second stage of labour had a lower incidence of perineal trauma (RR 0.61, 95% CI 0.42 to 0.88, Tau² = 0.08, I² = 78%) compared with no intervention, but had no clear effect on in the incidence of episiotomy (RR 0.79, 95% CI 0.44 to 1.42, Tau² = 0.16, I² = 70%) and first and second degree perineal lacerations (RR 0.58, 95% CI 0.31 to 1.10, Tau² = 0.18, I² = 59%). No side effects were reported in the included trials. No included trials reported on perineal pain and other pre-specified secondary outcomes: perineal trauma requiring suturing; blood loss; dyspareunia; urinary incontinence; faecal incontinence; assisted delivery rate; women's satisfaction; Apgar score less than seven at five minutes and need for admission to special care baby unit. Perineal HAase injection during second stage of labour had a lower incidence of perineal trauma compared with control or no intervention, but there was no clear evidence of benefit compared with placebo injection. The difference in incidence of perineal trauma may probably be due to bias and confounding in the non-placebo controlled comparison, this result should be interpreted cautiously. The potential use of perineal HAase injection as a method to reduce perineal trauma were yet to be determined as there was no appropriate established dose for HAase, no evidence of follow up, and the number of high-quality trials and outcomes reported were too limited to draw conclusions on its effectiveness and safety. Further rigorous randomised controlled trials are required to evaluate the role of perineal HAase injection in vaginal deliveries.

Plain-language summary: Two trials involving 283 women compared the effects of perineal HAase injection with placebo injection during second stage of labour and were at low risk of bias. Three trials (one three-armed trial was analysed twice) with 373 women compared the effects of perineal HAase injection during second stage of labour with no intervention. The overall results showed that perineal HAase injection had a significantly lower incidence of perineal trauma compared with control or no intervention, but there was no difference in the incidence of episiotomy, first and second degree and more severe (third and fourth degree) perineal tears. There was no clear evidence that HAase injection lowered the incidence of perineal trauma, episiotomy, first and second degree and more severe (third and fourth degree) perineal tears when compared with placebo injection. No side effects were reported in the included trials. Other measures such as perineal pain and other pre-specified secondary outcomes were not measured by the included trials. The difference in the incidence of perineal trauma may be due to bias and confounding in the non-placebo controlled comparison, this result should be interpreted cautiously. The potential use of this intervention as a method to reduce perineal trauma are yet to be determined as there was no appropriate established dose for HAase, no evidence of follow-up and side effects, and the number of high-quality trials and outcomes reported was too limited to draw conclusions on its effectiveness and safety. Further rigorous randomised controlled trials are required to evaluate the role of perineal HAase injection in vaginal deliveries.