Discussion
Our results support the increased risk of OASIS related to
well-established risk factors, including primiparity, instrumental
delivery and high birth weight. We showed an association with increasing
maternal age, obesity (if unadjusted for birth weight), lower maternal
height and previous CS. Smoking and low maternal education level were
negatively associated with OASIS.
Increasing BMI showed a decreased risk of OASIS in the crude analysis,
however only apparent in the adjusted model after adding birth weight to
the analysis. Obese women are more likely to give birth to larger babies
and it is therefore important to consider whether this apparent
protective effect is clinically relevant, or if their overall risk of
OASIS is instead increased secondary to risk of foetal macrosomia as
seen in the unadjusted results. A decreased risk of OASIS was seen in
two previous Swedish studies, with a more profound risk reduction in BMI
>35. Constable et al., showed a risk reduction (aOR 0.27)
among women with BMI >35 in an Australian setting, and
Garetto et al. observed an aOR 0.75 in obese American women. All
previously mentioned studies either adjusted for birth weight or
excluded cases with babies >4000g. On the contrary, other
studies have not been able to statistically detect any protective
effect, whereas Landy et al., only found a risk decrease among
nulliparous women.
The proposed underlying mechanisms of a protective effect include an
increased amount of adipose tissue making the perineal tissue softer and
more stretchable, and a larger perineal body increasing the anovaginal
distance. These hypotheses are still to be scientifically proven. In
contrast, perineal tissue has been suggested to be weaker in obese
women; higher BMI has been associated with striae gravidarum which in
turn has been linked to a higher risk of perineal trauma. Another
consideration is potentially increased missed diagnoses relating to
difficulties in examining women with voluminous amount of adipose
tissue.
Women with <9 years education had a 15% lower risk of
reported OASIS as compared to those with >3 years of higher
education after adjusting for maternal characteristics. Similar results
are seen in a Finnish study where higher socioeconomic status (SES) was
associated with an 18% increased risk of OASIS. A British study showed
that women of high SES were more than twice as likely to suffer from
OASIS as compared to those of low SES. This difference is somewhat
surprising and difficult to explain. It either stands for an unknown
confounding factor not accounted for, or potentially by a difference in
the provision of health care. The higher risk in women with higher
educational level could instead represent a lower risk of missed
diagnosis.
We found smoking to be negatively associated with OASIS (cRR 0.49),
remaining after adjusting for maternal characteristics, and surprisingly
also after adjusting for delivery mode and birth weight (aRR of 0.74).
Similar results are seen in a Finnish study where smoking decreased the
incidence of OASIS in primiparous women by 28%, and by 20% in a
previous Swedish study among primiparous women. The underlying
mechanisms of the apparent protective effect are yet unexplained. The
results appear paradoxical to other known effects of smoking such as
premature skin ageing, reduced blood flow, and an accumulation of
body-wide oxidative damage. This causes us to speculate around the true
protective effect of smoking, or whether a difference in provision of
health care can explain the results.
Maternal age was inversely related to risk of OASIS where women
<20 years of age had a 50% lower risk of OASIS compared to
women aged 24-29 years. Women >35 years on the other hand,
had an approximately 20% risk increase. The highest age group
(>40) had a slightly lower risk increase after adjustments
than women aged 34-39, but the difference between the risk estimates was
far from significant (p=0.20). Gurol-Urgandi et al., showed a similar
risk decrease in teenage mothers, but only a slight risk increase (aOR
1.07) in women aged 30-34. A previous Swedish register-based study
showed the risk of OASIS to increase almost continuously with age
irrespective of parity, with exception for nulliparous women where the
risk did not further increase >35 years which was similar
to our findings. In this previous study, the aOR was around 2 for
nulliparous women >30 years, however the reference group
differed from ours (<25 years) which could explain the
difference in calculated risks. Similarly, Hornemann et al. found a
higher maternal age in women with OASIS compared to women with less
severe perineal lacerations. The specific effects of ageing on perineal
tissue have been poorly researched and age-specific advantage of
protective measures have not been studied. Considering the trends in
modern society to start a family at an older age the risk should be
acknowledged, and attention should be given to the importance of
accurate diagnosis and treatment.
Maternal height was inversely related to risk of OASIS. Women
<155 cm had an aRR of 1.34 after adjusting for maternal
characteristics. We stipulated adjustments for delivery mode and birth
weight would lessen this association, but the correlation instead became
more profound and aRR increased to 1.72, suggesting maternal height as a
significant independent risk factor. Räisänen et al. found an increase
of 1 cm in maternal height to decrease the risk of sphincter injury by
2%. However, there is limited research on the effects of maternal
height on obstetric outcomes.
Our results show an increased risk of OASIS among women achieving VBAC
compared to primiparous women. D’Souza et al. found a similar risk of
OASIS after VBAC compared to that of primiparous women, and a 6.8-fold
risk increase in secundiparous women as compared to multiparous women
with previous vaginal births only. Räisänen et al. found a 1.4-fold risk
increase associated with VBAC and propose an underlying foetopelvic
disproportion as a potential explanation, putting women at risk of
initial CS and subsequently of OASIS in following vaginal deliveries.
Strengths of this study include using a population-based register
providing a large study population available for inclusion. This enabled
analysis of multiple risk factors to produce reliable results. The
Swedish MBR is of high quality with <2% missing data and is
mandatory across the population providing non-selective standardised
data. Nevertheless, the register contains some errors and missing
values. We also recognise that the classification and diagnosis of OASIS
is subjective and depends on the care provider and local routine.
Furthermore, our analysis did not include foetal head circumference,
length of second stage of labour, use of anaesthetic or episiotomy rates
which might impact the overall risk.