Discussion
In this unselected population, OASI was associated with known non-modifiable factors like high maternal age, first pregnancy, and large babies. Of potentially modifiable factors, OASI was associated with induction of labour and instrumental vaginal delivery in primiparous women, and with amniotomy and augmentation with oxytocin in both primi- and multiparous women, procedures that are primarily initiated to accelerate delivery.
The major strengths of this study were the unselected population, the large number of participants, and completeness of data. We also consider the inclusion of only one obstetric hospital a strength since no major official changes in routines were introduced, although we cannot exclude gradual unrecognized changes during this 13-year period. The retrospective nature of the study may be a weakness since reasons for performing amniotomy and augmentation with oxytocin were not necessarily specified and since vigilance in classifying perineal rupture may have been less accurate than in a planned prospective study. Furthermore, the women were not routinely followed after discharge from the hospital. The time lap between the collection and publication of data may make the results less valid of today’s practice since increased focus on reducing the incidence of OASI has been implemented since the data were collected (20, 23). However, the use of induction of labour and augmentation with oxytocin have increased nationally since these data were collected, and the risks related to these interventions may correspond to what we found (33).
To our knowledge, our study is the first to include amniotomy as a potential independent risk factor for OASI. Amniotomy was the strongest independent modifiable risk factor regardless of parity and suggests that attention to indications and timing of amniotomy may be a hitherto unrecognized means of preventing OASI. The use of amniotomy varies between institutions both in Norway and other countries and ranges from 20% to 60% (34, 35). However, in our experience the documentation of amniotomy in patient charts during labour is highly variable. Even though we have a national high-quality birth register in Norway, the use of amniotomy in spontaneous labour is not reported (33).
With the goal of reducing cesarean births through active management of labour, amniotomy has been widely and readily accepted to avoid labour for more than 12 hours (36). However, reducing length of labour might not be a benefit for all women, and a Cochrane review from 2013 concluded that there is no evidence to support routine amniotomy to shorten spontaneous labour or to avoid prolonged labour (37). The mechanism behind the association between amniotomy and OASI is unclear, but we speculate that amniotomy may disrupt the normal physiologic process of gradual adaptation of the birth canal and thereby a higher risk of trauma.
In the present study, we also found that augmentation with oxytocin was an independent risk factor for OASI for both primi- and multiparous women. This is in accordance with previous studies (1, 38). Augmentation with oxytocin is widely used when labour is delayed, and probably more than half of women in labour worldwide receive oxytocin augmentation (33, 35, 39). However, this varies widely between countries and within the same country. In our study, 60% of the primiparous and 46.7% of the multiparous women were augmented with oxytocin, which is in line with current rates in maternity wards in Norway (35). Increased and reduced control of contractions are known potential adverse effect of augmentation of labour with oxytocin (40). We suggest that the effects of augmentation with oxytocin are similar to that of amniotomy in that the birth progress may be more rapid than the natural adaptation of the birth canal.
Instrumental vaginal delivery is a well-established risk factor for OASI (1, 2, 6, 31). However, this was only an independent risk factor for primiparous women in our study. Instrumental delivery was also associated with OASI in multiparous women in the unadjusted analysis, and the reason for no significant association in the adjusted analysis may partly be that the study lacked power to detect a risk since instruments were rarely used in this group.
In conclusion, the study suggests that indications for and timing of amniotomy and augmentation of the birth process with oxytocin need to be readdressed in order to reduce the risk of severe perineal ruptures.