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\begin{document}

\title{Obstetric Anal Sphincter Injuries -- Maternal, Foetal and
Sociodemographic Risk Factors: A Retrospective Register-Based Study}



\author[1]{Kristin André}%
\author[2]{Andrea Stuart}%
\author[3]{Kärin Kallén}%
\affil[1]{Affiliation not available}%
\affil[2]{Helsingborgs lasarett}%
\affil[3]{Lund University Clinical Sciences}%


\vspace{-1em}



  
  \date{May 30, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
Objective. To determine risk and protective factors of obstetric anal
sphincter injuries (OASIS). Design. A retrospective register-based
observational study. Setting. Sweden. Population. A cohort of 988, 988
singleton term deliveries 2005-2016 were included. Methods. Data from
the Swedish Medical Birth Registry and Statistics Sweden were extracted
to identify cases of OASIS and maternal and foetal characteristics.
Modified Poisson Regression analyses were performed to assess risk
factors. Main outcome measures. Risk ratios for OASIS with 95\%
confidence interval associated with maternal and foetal risk factors
were calculated. Results. The rate of OASIS was 3.5\% (n=34, 583).
Primiparity (aRR 3.13 95\% CI 3.05--3.21), vacuum extraction (aRR 2.79
95\% CI 2.73--2.86), forceps (aRR 4.27 95\% CI 3.86--4.72) and high
birth weight (aRR 2.61 95\% CI 2.50--2.72) were associated with a
significantly increased risk of OASIS. Increasing maternal age and
decreasing maternal height increased the risk of OASIS. Smoking (aRR
0.74 95\% CI 0.70--0.79) and low maternal education (aRR 0.87 95\% CI
0.83--0.92) were associated with a decreased frequency of reported
OASIS. Obesity decreased the risk of OASIS (aRR 0.90 95\% CI
0.87--0.94), but only after adjusting for foetal birth weight. Previous
caesarean section increased the risk of OASIS (aRR 1.41; 95\% CI
1.36--1.47). Conclusion. Primiparity, instrumental delivery and high
birth weight increased the risk of OASIS. Risk factors including BMI,
height, age, smoking, maternal education, ethnicity and previous
caesarean section also contribute to the overall risk of OASIS.
Keywords. Obstetric sphincter injuries, risk factors, pregnancy.%
\end{abstract}%



\sloppy


\textbf{Obstetric Anal Sphincter Injuries -- Maternal, Foetal and
Sociodemographic Risk Factors: A Retrospective Register-Based Study}

\selectlanguage{ngerman}\textbf{Kristin André}

Department of Obstetrics and Gynaecology, Helsingborg Central Hospital,
Helsingborg, Sweden

\textbf{Andrea Stuart}

Department of Obstetrics and Gynaecology, Helsingborg Central Hospital,
Helsingborg, Sweden

Institution of Clinical Sciences Lund, Lund University, Lund, Sweden

\textbf{Karin Källén}

Institution of Clinical Sciences Lund, Lund University, Lund, Sweden

Centre for Reproductive Epidemiology, Tornblad Institute, Lund
University, Lund, Sweden

\textbf{Corresponding author:}

Kristin André

Department of Obstetrics and Gyneacology, Helsingborgs Lasarett.

Charlotte Yhléns gata 10, 25223 Helsingborg, Sweden

+46734233405

kristin.c.andre@gmail.com

\textbf{Running title:} Risk Factors for Obstetric Anal Sphincter
Injuries



\textbf{Abstract}

\textbf{Objective} . To determine risk and protective factors of
obstetric anal sphincter injuries (OASIS). \textbf{Design} . A
retrospective register-based observational study. \textbf{Setting} .
Sweden. \textbf{Population} . A cohort of 988,~988 singleton term
deliveries 2005-2016 were included. \textbf{Methods} . Data from the
Swedish Medical Birth Registry and Statistics Sweden were extracted to
identify cases of OASIS and maternal and foetal characteristics.
Modified Poisson Regression analyses were performed to assess risk
factors. \textbf{Main outcome measures} . Risk ratios for OASIS with
95\% confidence interval associated with maternal and foetal risk
factors were calculated. \textbf{Results} . The rate of OASIS was 3.5\%
(n=34, 583). Primiparity (aRR 3.13 95\% CI 3.05--3.21), vacuum
extraction (aRR 2.79 95\% CI 2.73--2.86), forceps (aRR 4.27 95\% CI
3.86--4.72) and high birth weight (aRR 2.61 95\% CI 2.50--2.72) were
associated with a significantly increased risk of OASIS. Increasing
maternal age and decreasing maternal height increased the risk of OASIS.
Smoking (aRR 0.74 95\% CI 0.70--0.79) and low maternal education (aRR
0.87 95\% CI 0.83--0.92) were associated with a decreased frequency of
reported OASIS. Obesity decreased the risk of OASIS (aRR 0.90 95\% CI
0.87--0.94), but only after adjusting for foetal birth weight. Previous
caesarean section increased the risk of OASIS (aRR 1.41; 95\% CI
1.36--1.47). \textbf{Conclusion} . Primiparity, instrumental delivery
and high birth weight increased the risk of OASIS. Risk factors
including BMI, height, age, smoking, maternal education, ethnicity and
previous caesarean section also contribute to the overall risk of
OASIS.\textbf{Keywords} . Obstetric sphincter injuries, risk factors,
pregnancy.

\textbf{Tweetable abstract}

Primiparity, instrumental delivery and VBAC increased the risk of OASIS,
whilst maternal smoking and low educational level was associated with
lower incidences of reported OASIS.



\textbf{Introduction}

Obstetric anal sphincter injuries (OASIS) are serious complications to
vaginal deliveries. They represent the most significant risk factor for
developing female anal incontinence and might also cause perineal pain,
dyspareunia, faecal urgency and urinary incontinence, leading to
significant long-term suffering and potential stigmatisation.

Perineal injuries are classified according to a definition published by
Sultan. First and second-degree injuries involve the vaginal epithelium
and perineal muscles respectively. OASIS include third- and
fourth-degree injuries to the perineum involving the external anal
sphincter (EAS), internal anal sphincter (IAS), and the anal epithelium
to varying extent.

There are several well-established risk factors associated with a higher
risk of OASIS, including primiparity, foetal macrosomia, instrumental
vaginal delivery, and prolonged second stage of delivery. Other aspects
that have been proposed to affect the risk include maternal age, height,
weight, smoking, ethnicity and educational level, but the evidence has
either been sparse or not consistent in published material.

The prevalence of sphincter injury has fluctuated over the last decades
and varies across countries. The incidence varies between 0.7-4.2\% in
the Nordic countries, with Finland representing the lowest and Denmark
the highest numbers. An increasing trend has been observed where several
explanations have been proposed; larger infant size, increased maternal
age, increase in instrumental deliveries, improved diagnostic accuracy,
and changed clinical practice of management of second stage of delivery.

Despite OASIS being recognised as a significant cause of both long-, and
short-term morbidity there is no international consensus regarding the
use of preventative measures. Several factors modifiable during labour
have been investigated. Controlled delivery of the head, warm compresses
held against the perineum, and choice of delivery position all have
shown to decrease the risk of OASIS. The evidence regarding perineal
protection manoeuvres has not been proven to have an effect in clinical
studies but is used on a routine basis in clinical practice in Sweden
and has been part of interventional programs that have successfully
reduced the incidence of OASIS. For women at very high risk of OASIS a
planned caesarean section might be offered as a preferred route of
delivery.

Identifying risk factors in order to provide optimal preventative
measures for high-risk women could lead to a significant decrease in
long-term suffering and stigmatised symptoms. The aim of this nationwide
register-based study was therefore to assess maternal, foetal and
socio-demographic risk and protective factors for OASIS across the
Swedish population.



\textbf{Methods and materials}

The study population consisted of 988,988 women with singleton term
(\selectlanguage{english}[?]37+0) vaginal deliveries in Sweden 2005-2016. The data was extracted
from the Swedish Medical Birth Registry (MBR), which was founded in 1973
and includes data on almost all deliveries (98-99\%) in Sweden. All
health care providers are obliged to report to the registry and the
system is identical throughout the country. It contains information
collected from medical records registered at antenatal visits, delivery
units and initial paediatric examination of the infant. A detailed
quality control and validation of the registry was published in 2003.

At the first antenatal visit the women are interviewed by their midwife
regarding their pre-pregnancy health, including smoking habits and use
of medical drugs. Also, the height and their booking weight are
registered. Maternal Body Mass Index (BMI) was calculated through weight
(kg)/(height (m))\textsuperscript{2} , and was classified according to
WHO`s classification into underweight (BMI \textless{}18.5), normal
weight (BMI 18.5 -- 24.9), overweight (BMI 25-29.9), and obesity (BMI \selectlanguage{english}[?]
30). Information regarding maternal educational level and maternal
country of birth was obtained through linkage with data from Statistics
Sweden. Cases of OASIS were identified through checkboxes, or ICD-code
O702 and O703 reported to the MBR or the National Patient Registry (NPR)
within six weeks after delivery.

\textbf{Statistics}

Risk Ratios (RR) and Adjusted Risk Ratios (aRR) for sphincter rupture
were obtained using modified Poisson Regression analyses (SPSS version
25). When specified, adjustments were made for year of delivery (classes
2005--2008, 2009--2012, 2013--2016) and maternal characteristics:
Maternal age (classes \textless{}20, 20--24.9, 25--29.9, 30--34.9,
35--39.9, or \selectlanguage{english}[?]40 years), parity (1, 2, or \selectlanguage{english}[?]3 children), BMI classes
(\textless{}18.5, 18.5--24.9, 25--29.9, \selectlanguage{english}[?]30, or not known), smoking
(yes, no, or not known), maternal height classes (\textless{}155,
155-164, 165-174, \selectlanguage{english}[?]175 cm, or not known), maternal country of birth
(Nordic countries, other European countries/USA/Canada/Australia/New
Zeeland, others, or not known), and maternal educational level
(\selectlanguage{english}[?]primary/lower secondary (1-9 years), upper secondary (10-12 years), \selectlanguage{english}[?]3
years higher education, \selectlanguage{english}[?]3 years higher education, or not known). When
specified, adjustments were also made for delivery mode (vaginal
non-instrumental, vacuum extraction, or forceps), and/or infant birth
weight (\textless{}2500, 2500-2999, 3000-3499, 3500-3999, 4000-4499,
\selectlanguage{english}[?]4500g). Findings with p-values \textless{}0.05 were regarded as
statistically significant.



\textbf{Results}

Between 2005-2016, 988 988 vaginal deliveries were recorded in the MBR.
The rate of OASIS was 3.5\% (n=34~583). Table 1 shows maternal
characteristics and delivery mode in relation to the risk of OASIS.
There was an overall seven percent decrease of OASIS during 2013-2016 as
compared to 2005-2008.

The risk of OASIS increased with maternal age as seen after adjusting
for maternal characteristics (Table 2), with the highest risk in the age
group 34-39 years, and inversely young maternal age of \textless{}20
years was significantly protective. When also adjusting for delivery
mode, the association between OASIS and increasing maternal age was less
pronounced, albeit still statistically significant.

Primiparous women were approximately three times as likely to attain
OASIS as compared to multiparous women, and the strong association
between primiparity and OASIS remained after adjustments.

In the crude model, high BMI (\textgreater{}30) had a considerably
protective effect (cRR 0.80), but after initial adjustment for maternal
characteristics and then delivery mode this association disappeared.
After also adjusting for birth weight a negative association was again
seen, but less pronounced (aRR 0.90). Low BMI (\textless{}18.5) showed a
risk increase of OASIS as compared to women with normal BMI (cRR 1.12).
Through all three steps of adjustment the risk increase among women with
BMI \textless{}18.5 was maintained, and the positive association was
even more noticeable (aRR 1.30). Maternal height showed an inverted
relationship to risk of OASIS which became more evident after adjusting
both for maternal characteristics, delivery mode and birth weight
(height \textless{}155 cm: aRR 1.72, compared to height
\textgreater{}175 cm: aRR 0.82).

Maternal smoking appeared to be negatively associated with risk of OASIS
that remained after adjusted calculations, even when adjusting for birth
weight (aRR 0.74). An association between increased risk of OASIS and
higher maternal educational level was found, and adjustment for maternal
characteristics, delivery mode, and birth weight only marginally altered
the risk estimates. According to the crude estimates, no association
between maternal country of birth and OASIS was found, but when
adjustments were made, decreased risk (aRR 0.92) was found for women
from Europe/ USA/ Canada/ Australia/ New Zeeland as compared to women
from the Nordic countries, whereas women with other ethnic origin had a
significant risk increase (aRR 1.31) as compared to women from the
Nordic countries.

The strongest risk factor for OASIS was instrumental delivery, where the
most significant risk increase was seen with the use of forceps (cRR
6.88). The prevalence of vacuum extraction (VE) was 8.3\% and the use of
forceps was rare at 0.2\%.

Vaginal birth after previous caesarean section (VBAC) as a risk factor
of OASIS was analysed separately and the risk of OASIS among
secundiparous women with a vaginal birth following one previous
caesarean section (2,755/29,927) was compared to the corresponding risk
among primiparous women with vaginal births (25,576/429,727). This
analysis showed a profound risk increase (cRR 1.60; 95\% CI 1.54--1.66)
of OASIS after previous CS. After adjustment for maternal
characteristics the risk of OASIS remained significantly elevated (aRR
1.41; 95\% CI 1.36--1.47). This risk increase could partly be explained
by an increase in instrumental delivery, but after adjustment for
delivery mode and birth weight the risk remained raised (aRR 1.24; 95\%
CI 1.20--1.29).

\textbf{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
\textgreater{}35. Constable et al., showed a risk reduction (aOR 0.27)
among women with BMI \textgreater{}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 \textgreater{}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 \textless{}9 years education had a 15\% lower risk of
reported OASIS as compared to those with \textgreater{}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
\textless{}20 years of age had a 50\% lower risk of OASIS compared to
women aged 24-29 years. Women \textgreater{}35 years on the other hand,
had an approximately 20\% risk increase. The highest age group
(\textgreater{}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 \textgreater{}35 years which was similar
to our findings. In this previous study, the aOR was around 2 for
nulliparous women \textgreater{}30 years, however the reference group
differed from ours (\textless{}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
\textless{}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. Raisanen 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. Raisanen 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 \textless{}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.

\textbf{Conclusion}

In this large register-based study we observed an increased risk of
OASIS associated with primiparity, instrumental delivery, increasing
maternal age, previous CS and low maternal height. Obesity was observed
to slightly increase the risk of OASIS if adjustments for birth weight
were not made, which might be the most clinically appropriate adjustment
model to use in this case. A negative association with OASIS was seen
with smoking and low maternal educational levels, however there are
uncertainties whether this association represents a true protective
effect or a potential difference in the provision of health care. Future
studies will be able to use these results in a clinical setting aimed at
individual risk assessment improving women's health.

\textbf{Acknowledgements} : None

\textbf{Disclosure of interest} : None to declare

\textbf{Contributions of authorship} : KA, AS and KK contributed to the
overall design of the study and the analysis and interpretation of the
data. KA drafted the first manuscript. AS and KK then revised and
critically appraised the draft before all authors approved of the final
manuscript.

\textbf{Details of ethics approval} : The study was approved by the
Regional Medical Ethics Committee, Lund, Sweden (2018/538).

\textbf{Funding} : None to declare

\textbf{References}

Table 1. Maternal characteristics and delivery mode in relation to the
risk of OASIS\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
& & OASIS & OASIS & All vaginal births & Crude & Crude\tabularnewline
\midrule
\endhead
& & n & (\%) & n & RR & (95\%CI)\tabularnewline
Year of delivery & 2005--2008 & 12 226 & (3.6) & 335 805 & 1.0 &
Reference\tabularnewline
& 2009--2012 & 12 370 & (3.5) & 357 174 & 0.95 &
0.93--0.98\tabularnewline
& 2013--2016 & 9987 & (3.4) & 296 009 & 0.93 & 0.90--0.95\tabularnewline
Maternal age (years) & \textless{}20 & 349 & (2.3) & 15 423 & 0.59 &
0.53--0.66\tabularnewline
& 20--24 & 3959 & (3.0) & 133 787 & 0.77 & 0.74--0.80\tabularnewline
& 24--29 & 11 522 & (3.8) & 300 127 & 1.0 & Reference\tabularnewline
& 30--34 & 13 037 & (3.8) & 341 750 & 0.99 & 0.97--1.02\tabularnewline
& 34--39 & 4919 & (3.0) & 165 330 & 0.78 & 0.75--0.80\tabularnewline
& [?]40 & 797 & (2.4) & 32 571 & 0.64 & 0.59--0.68\tabularnewline
Parity & 1 & 25 576 & (6.0) & 429 727 & 2.81 & 2.74--2.88\tabularnewline
& 2 & 7871 & (2.1) & 371 933 & 1.0 & Reference\tabularnewline
& [?]3 & 1136 & (0.6) & 187 338 & 0.29 & 0.27--0.31\tabularnewline
Smoking & Yes & 1077 & (1.8) & 60340 & 0.49 & 0.47-0.52\tabularnewline
& No & 31891 & (3.6) & 882298 & 1.0 & Reference\tabularnewline
& Not known & 1615 & (3.5) & 46360 & 0.96 & 0.92-1.01\tabularnewline
Body Mass Index (kg/m\textsuperscript{2}) & \textless{}18.5 & 951 &
(4.0) & 23543 & 1.12 & 1.05-1.19\tabularnewline
& 20-24.9 & 20428 & (3.6) & 563735 & 1.0 & Reference\tabularnewline
& 25-29.9 & 7563 & (3.4) & 222729 & 0.94 & 0.91-0.96\tabularnewline
& [?]30 & 3022 & (2.9) & 103740 & 0.80 & 0.77-0.84\tabularnewline
& Not known & 2600 & (3.5) & 74627 & 0.96 & 0.92-1.00\tabularnewline
Maternal height (cm) & \textless{}155 & 1159 & (4.2) & 27892 & 1.21 &
1.14-1.28\tabularnewline
& 155-164 & 11704 & (3.6) & 325729 & 1.05 & 1.02-1.07\tabularnewline
& 165-174 & 16745 & (3.4) & 488258 & 1.0 & Reference\tabularnewline
& [?]175cm & 3332 & (3.3) & 99815 & 0.97 & 0.94-1.01\tabularnewline
& Not known & 1643 & (3.5) & 47298 & 1.01 & 0.96-1.07\tabularnewline
Maternal educational level & [?]Primary/lower secondary & 1972 & (2.3) &
86794 & 0.55 & 0.52-0.57\tabularnewline
& Upper secondary & 9675 & (2.9) & 333887 & 0.70 &
0.68-0.72\tabularnewline
& \textless{}3 yrs higher education & 4280 & (3.6) & 119622 & 0.86 &
0.83-0.89\tabularnewline
& [?]3 yrs higher education & 15999 & (4.2) & 385437 & 1.00 &
Reference\tabularnewline
& Not known & 2657 & (4.2) & 63258 & 1.01 & 0.97-1.05\tabularnewline
Maternal country of birth & Nordic countries & 30112 & (3.5) & 860890 &
1.00 & Reference\tabularnewline
& Europe/USA/ Canada/Australia/NZ & 698 & (3.4) & 20409 & 0.98 &
0.97-1.04\tabularnewline
& Other & 3773 & (3.5) & 107699 & 1.00 & 0.97-1.05\tabularnewline
Delivery mode & Non-instrumental & 23946 & (2.6) & 905290 & 1.00 &
Reference\tabularnewline
& VE & 10314 & (12.6) & 81932 & 4.76 & 4.66-4.87\tabularnewline
& Forceps & 323 & (18.2) & 1776 & 6.88 & 6.23-7.59\tabularnewline
\bottomrule
\end{longtable}

Table 2. Maternal risk factors for OASIS after multivariable modified
Poisson regression analyses.\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
& & Adjusted (maternal characteristics (MC)) & Adjusted (maternal
characteristics (MC)) & Adjusted (MC + delivery mode (DM)) & Adjusted
(MC + delivery mode (DM)) & Adjusted (MC + DM + birth weight) & Adjusted
(MC + DM + birth weight)\tabularnewline
\midrule
\endhead
& & RR & (95\%CI) & RR & (95\%CI) & RR & (95\%CI)\tabularnewline
Year of delivery & 2005--2008 & 1.00 & Reference & 1.00 & Reference &
1.00 & Reference\tabularnewline
& 2009--2012 & 0.94 & 0.92--0.97 & 0.96 & 0.94--0.98 & 0.96 &
0.93--0.98\tabularnewline
& 2013--2016 & 0.92 & 0.90--0.95 & 0.97 & 0.94--0.99 & 0.96 &
0.94--0.99\tabularnewline
Maternal age (years) & \textless{}20 & 0.45 & 0.40--0.50 & 0.50 &
0.45--0.56 & 0.49 & 0.44--0.55\tabularnewline
& 20--24 & 0.67 & 0.65--0.70 & 0.72 & 0.69--0.74 & 0.70 &
0.68--0.73\tabularnewline
& 24--29 & 1.0 & Reference & 1.00 & Reference & 1.00 &
Reference\tabularnewline
& 30--34 & 1.24 & 1.21--1.27 & 1.18 & 1.15--1.21 & 1.20 &
1.17--1.23\tabularnewline
& 34--39 & 1.29 & 1.25--1.34 & 1.18 & 1.14--1.22 & 1.23 &
1.19--1.27\tabularnewline
& [?]40 & 1.23 & 1.15--1.32 & 1.09 & 1.01--1.17 & 1.18 &
1.10--1.26\tabularnewline
Parity & 1 & 3.13 & 3.05--3.21 & 2.47 & 2.40--2.53 & 2.96 &
2.88--3.04\tabularnewline
& 2 & 1.0 & Reference & 1.00 & Reference & 1.00 &
Reference\tabularnewline
& [?]3 & 0.27 & 0.25--0.29 & 0.29 & 0.27--0.31 & 0.27 &
0.26--0.29\tabularnewline
Smoking & Yes & 0.64 & 0.60--0.68 & 0.65 & 0.61--0.69 & 0.74 &
0.70--0.79\tabularnewline
& No & 1.0 & Reference & 1.00 & Reference & 1.00 &
Reference\tabularnewline
& Not known & 0.95 & 0.88--1.03 & 0.96 & 0.88--1.04 & 0.96 &
0.89--1.04\tabularnewline
Body Mass Index (kg/m\textsuperscript{2}) & \textless{}18.5 & 1.12 &
1.05--1.20 & 1.11 & 1.04--1.18 & 1.30 & 1.22--1.38\tabularnewline
& 20--24.9 & 1.0 & Reference & 1.00 & Reference & 1.00 &
Reference\tabularnewline
& 25--29.9 & 1.07 & 1.04--1.10 & 1.06 & 1.04--1.09 & 0.97 &
0.94--0.99\tabularnewline
& [?]30 & 1.04 & 1.04--1.10 & 1.04 & 1.00--1.08 & 0.90 &
0.87--0.94\tabularnewline
& Not known & 1.01 & 0.95--1.08 & 1.02 & 0.95--1.08 & 0.98 &
0.92--1.04\tabularnewline
Maternal height (cm) & \textless{}155 & 1.34 & 1.26--1.42 & 1.24 &
1.17--1.31 & 1.72 & 1.62--1.82\tabularnewline
& 155--164 & 1.11 & 1.08--1.13 & 1.07 & 1.04--1.09 & 1.23 &
1.21--1.26\tabularnewline
& 165--174 & 1.0 & Reference & 1.00 & Reference & 1.00 &
Reference\tabularnewline
& [?]175 & 0.92 & 0.88--0.95 & 0.94 & 0.91--0.98 & 0.82 &
0.79--0.85\tabularnewline
& Not known & 1.03 & 0.93--1.14 & 1.01 & 0.92--1.12 & 1.09 &
0.99--1.21\tabularnewline
Maternal education level & [?]Primary/lower secondary & 0.85 & 0.81--0.89
& 0.84 & 0.80--0.88 & 0.87 & 0.83--0.92\tabularnewline
& Upper secondary & 0.88 & 0.85--0.90 & 0.87 & 0.85--0.89 & 0.87 &
0.85--0.90\tabularnewline
& \textless{}3 yrs higher education & 0.92 & 0.89--0.95 & 0.92 &
0.89--0.95 & 0.92 & 0.90--0.96\tabularnewline
& [?]3 yrs higher education & 1.00 & Reference & 1.00 & Reference & 1.00 &
Reference\tabularnewline
& Not known & 0.94 & 0.90--0.98 & 0.93 & 0.89--0.97 & 0.95 &
0.91--0.98\tabularnewline
Maternal country of birth & Nordic countries & 1.00 & Reference & 1.00 &
Reference & 1.00 & Reference\tabularnewline
& Europe/USA/ Canada/Australia/NZ & 0.91 & 0.84--0.98 & 0.91 &
0.84--0.98 & 0.92 & 0.86--0.99\tabularnewline
& Other & 1.21 & 1.17--1.25 & 1.20 & 1.16--1.24 & 1.31 &
1.26--1.35\tabularnewline
Delivery mode & Non-instrumental & & & & & 1.00 &
Reference\tabularnewline
& VE & & & & & 2.79 & 2.73--2.86\tabularnewline
& Forceps & & & & & 4.27 & 3.86--4.72\tabularnewline
\bottomrule
\end{longtable}

Table 3. Birth weight in relation to risk of sphincter rupture\selectlanguage{english}
\begin{longtable}[]{@{}llllllllll@{}}
\toprule
& Sphincter rupture & Sphincter rupture & All vaginal births & Crude &
Crude & Adjusted (maternal characteristics) & Adjusted (maternal
characteristics) & Adjusted (delivery mode) & Adjusted (delivery
mode)\tabularnewline
\midrule
\endhead
Birth weight (g) & n & (\%) & N & OR & (95\%CI) & OR & (95\%CI) & OR &
(95\%CI)\tabularnewline
\textless{}2500 & 231 & (0.9) & 25 472 & 0.24 & 0.21--0.27 & 0.17 &
0.15--0.20 & 0.19 & 0.17--0.22\tabularnewline
2500--2999 & 1845 & (1.8) & 101 202 & 0.48 & 0.45--0.50 & 0.36 &
0.34--0.38 & 0.38 & 0.36--0.40\tabularnewline
3000--3499 & 8641 & (2.7) & 323 256 & 0.70 & 0.68--0.72 & 0.59 &
0.57--0.60 & 0.61 & 0.60--0.63\tabularnewline
3500--3999 & 13 713 & (3.8) & 357 012 & 1.00 & ref & 1.00 & Ref & 1.00 &
Ref\tabularnewline
4000--4499 & 7829 & (5.2) & 149 762 & 1.36 & 1.33--1.40 & 1.65 &
1.60--1.69 & 1.56 & 1.52--1.60\tabularnewline
[?]4500 & 2324 & (7.2) & 32 284 & 1.87 & 1.80--1.96 & 2.84 & 2.73--2.96 &
2.61 & 2.50--2.72\tabularnewline
\bottomrule
\end{longtable}

\selectlanguage{english}
\FloatBarrier
\end{document}

