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.