Risk factors for perineal and vaginal tears in primiparous women – a prospective cohort study

Background The aim of this study was to estimate the incidence of second-degree perineal tears, obstetric anal sphincter injuries, and high vaginal tears in primiparous women, and to examine how sociodemographic and pregnancy characteristics, hereditary factors, obstetric management and the delivery process are associated with the incidence of these tears. Methods All nulliparous women registering at the maternity health care in Region Örebro County, Sweden, in early pregnancy between 1 October 2014 and 1 October 2017 were invited to participate in a prospective cohort study. Data on maternal and obstetric characteristics were extracted from questionnaires completed in early and late pregnancy, from a study-specific delivery protocol, and from the obstetric record system. These data were analyzed using unadjusted and adjusted multinomial and logistic regression models. A total of 644 women were included in the study sample. Fetal weight exceeding 4000 g and vacuum extraction were found to be independent risk factors for both second-degree perineal tears (adjusted odds ratio 2.26 (95% CI: and 3.28 (95% CI: respectively) and OASI (adjusted odds ratio 5.93 (95% CI: and 4.31 (95% CI: respectively), whereas age above 25 years only increased the risk of second-degree tears (adjusted (95% CI: Heredity of pelvic floor dysfunction and/or and fetal exceeding 35 cm were independent risk factors for tears (adjusted odds ratio 2.33 (95% CI: 3.64 (95% CI: 1.54, 8.59) and 2.86 (95% CI: 1.43, 5.75) respectively). connective tissue deficiency, induced labor, and fetal head circumference > 35 cm. These tears have been found to be common among primiparous women, and merit more attention. Brief summary: This prospective cohort study showed that fetal birthweight exceeding 4000 g and vacuum extraction increased the risk of second-degree tear and anal sphincter injury. incontinence, pelvic pelvic to pelvic prolapse, urinary incontinence, inguinal hernia, or varicose veins. Stress urinary incontinence was as reporting urine leakage “often” or “sometimes” during physical strain. Symptoms of pelvic was as responding “often” or “sometimes” to the about the sensation of vaginal


Introduction
Perineal tears affect about 80% of women during childbirth, with primiparous women being affected more frequently than multiparous women [1,2]. The rate of second-degree perineal tears, which involves the vagina and/or perineal muscle, has been reported in English and Swedish studies to be 35.1-78.3% among primiparous women and 34. .6% among multiparous women [1-3], while third-and fourth-degree tears, which involve varying degrees of injury to the anal sphincters, occur in 5.1-6.6% of primiparous women and 1.8-2.7% of multiparous women [1,2]. Between 1990 and 2016, the incidence of third-and fourth-degree perineal tears among primiparous women in Sweden rose from 2.9-5.1% [4].
Obstetric anal sphincter injuries (OASI) are the largest obstetric risk factor for developing anal incontinence in women [5], so these tears merit particular attention. However, although less attention has been paid, seconddegree tears alone may impair sexual function [7] and increase the risk of future pelvic organ prolapse [8], and high vaginal tears have been associated with increased risk for levator muscle avulsion [9]. But the incidence and risk factors of these tears have been poorly investigated.
Various interventions have been attempted to prevent perineal tears, but few have been proven to reduce the incidence of severe perineal tears. There is moderate-quality evidence that warm compresses applied to the perineum during delivery and perineal massage can reduce the risk of OASI [9]. Episiotomy has been shown to be protective against OASI in instrumental vaginal delivery [10,11], but in spontaneous vaginal delivery the risk of severe perineal trauma is lower when episiotomy is used restrictively rather than routinely [12]. Randomized controlled studies have not shown any advantage of manual perineal support in reducing OASI [9]. An educational program developed in Finland including a specific technique of manual perineal support and mediolateral episiotomy on indication has been introduced in many obstetrics units in the Nordic countries, but the evidence for this intervention is extremely limited [13]. A non-randomized study from Sweden showed that a multifaceted intervention consisting of spontaneous pushing, birth positions with flexibility in the sacroiliac joints, and a two-step head-to-body delivery significantly reduced second degree tears, but these results have not yet been reproduced [3]. There is a need for new interventions to prevent severe perineal tears, and one way to approach such measures is epidemiologic research regarding risk factors for perineal tears.
Instrumental delivery [14,15], protracted second stage of labor [14,16], birth weight greater than 4 kg [16], and fetal occipito-posterior presentation [15,16] have been shown to be independent risk factors for OASI in several retrospective studies. Retrospective studies have generally focused on OASI, whereas second-degree tears have almost exclusively been reserved for prospective observational studies. Only six articles based on prospective observation studies of OASI and/or other perineal tears were identified in an extensive PubMed search [1,2,[17][18][19][20], and only two of these articles included second-degree perineal tears [1,2].
The aim of this study was to estimate the incidence of second-degree perineal tears, OASI (defined as any third or fourth degree perineal tear), and high vaginal tears in primiparous women, and to examine how sociodemographic and pregnancy characteristics, hereditary factors, obstetric management and the delivery process are associated with the incidence of these tears.

Study design and population
We conducted a prospective cohort study in the Region Örebro County, Sweden, named the Pelvic Floor In Pregnancy And Childbirth (POPRACT) study. All eligible nulliparous women registering for maternity health care in early pregnancy between 1 October 2014 and 1 October 2017 were informed about the study and asked if they wanted to participate by the midwife in charge. Antenatal care is free of charge in Sweden, and almost all women attend maternity health care. Exclusion criteria were first visit at maternity health care after 15 weeks + 6 days of gestation or insufficient knowledge of the Swedish language to complete the questionnaires used in the study. Participants were asked to complete web-based questionnaires on four occasions: at entry into the study in early pregnancy, at 36 weeks of gestation, at 8 weeks postpartum, and at 1 year postpartum. Patientreported data were managed in the cloud-based tool esMaker 3.0 (Entergate AB, Sweden) in accordance with the General Data Protection Regulation of the European Union. The questionnaires included items on general health, socioeconomic status, heredity of pelvic floor dysfunction and connective tissue deficiency, and self-reported pelvic floor dysfunction [21,22] (see Additional file 1).

Study size
Given that the expected incidence of OASI was > 7% and the expected incidence of second-degree perineal tear was significantly higher, a study population of 1000 women was judged to be sufficient to identify risk factors of clinical importance.

Exposure measures
The following patient-reported data from the first and second questionnaires (i.e. before delivery) were analyzed as potential risk factors for perineal tears and vaginal tear: level of education, heredity of pelvic floor disorders and/or connective tissue deficiency, symptoms of stress urinary incontinence, and symptoms of pelvic organ prolapse. Heredity of pelvic floor disease was defined as mother or sister having undergone surgery due to pelvic organ prolapse, urinary incontinence, inguinal hernia, or varicose veins. Stress urinary incontinence was defined as reporting urine leakage "often" or "sometimes" during physical strain. Symptoms of pelvic organ prolapse was defined as responding "often" or "sometimes" to the question about the sensation of vaginal bulging.
Participating women had their delivery at either of the two delivery wards in Region Örebro County, which are located at Örebro University Hospital and at Karlskoga Hospital. Delivery was assisted by a midwife under ordinary circumstances or by an obstetrician in case of instrumental delivery. Diagnosis of first-and seconddegree perineal tears was made by a midwife. In cases of suspected third-or fourth-degree perineal tear or a high vaginal tears, an obstetrician was consulted for an assessment and suturing. After delivery, vaginal examination, and suturing if necessary, the midwife (in co-operation with the obstetrician when needed) completed a study protocol containing specific questions about delivery characteristics, perineal and vaginal tears, and suturing. The part of the protocol regarding perineal tears and suturing has been validated in a previous study [23]. The protocol uses the Royal College of Obstetricians and Gynaecologists classification of perineal tears [24] and the ICD-10 classification of high vaginal tear; that is, a vaginal tear extending above the distal third of the vagina [25]. Information regarding oxytocin augmentation during active second stage of labor, use of perineotomy, manual perineal protection, and application of fetal scalp electrode was retrieved from the mentioned study protocol. In case of perineotomy the perineal tear was classified as second-degree at minimum. In women who had both episiotomy and a perineal tear of third or fourth degree, the classification of perineal tear remained unchanged. Perineotomy was not studied as a risk factor, since second-degree tears were included as an outcome in the multinomial regression and so such an analysis would have been inappropriate. Data concerning BMI and smoking at maternity health care registration, maternal age at delivery, gestational age at birth, whether delivery started spontaneously or was induced, administration of epidural analgesia, duration of active second stage of labor, maternal position at birth, mode of delivery, fetal presentation, fetal birth weight, and fetal head circumference were extracted from the obstetric record system (Obstetrix version 2.16.0.200, Cerner Corporation, Sweden) using an accessory program (Obstetrix Förlossningsliggare version 2.16.0.200, Cerner Corporation, Sweden). Variables were categorized as follows: age was categorized into ≤ 25 years and > 25 years; BMI into ≤ 25 kg/m 2 and 25.1-30 kg/m 2 ; gestational age at delivery into preterm (< 37 + 0), term (37 + 0-42 + 0), and postterm (> 42 + 0); duration of active second stage of labor into ≤ 15 min, 16-60 min, and > 60 min; mode of delivery into spontaneous and vacuum extraction; fetal presentation into occiput anterior and occiput posterior; fetal weight into ≤ 4000 g and > 4000 g; and fetal head circumference into ≤ 35 cm and > 35 cm. Maternal position at birth was categorized into lithotomy position and other positions, with the latter including squatting, kneeling, supine, lateral recumbent, sitting, standing, and undefined.

Outcome measures
The primary outcome measure was perineal tear, which was divided into three groups: 1) intact perineum or first degree tear (defined as the reference category), 2) second-degree tear, and 3) third-or fourth-degree tear, i.e. OASI. The secondary outcome measure was vaginal tear, which was categorized into two groups: 1) no or low vaginal tear (the reference category) and 2) high vaginal tear.

Statistical analyses
Relationships between potential risk factors and different degrees of perineal and vaginal tears were evaluated using unadjusted and adjusted multivarable regression models. Multinomial logistic regression was used for perineal tears, and logistic regression was used for vaginal tears. In the multivariate models for perineal tear, all potential risk factors were entered in the model and mutually adjusted for except heredity of pelvic floor dysfunction and/or connective tissue deficiency, stress urinary incontinence, perineotomy, whether hand or arm was the presenting part, and fetal head > 35 cm. In the case of vaginal tear, all risk factors except stress urinary incontinence and fetal weight > 4000 g were entered in the adjusted model. Assessment of potential multicollinearity among risk factors showed no collinearity issues; all variance inflation factors were < 1.6. An interaction between fetal weight and delivery mode on the risk of perineal tear was examined using interaction tests. Differences between vaginally delivered women with and without a registered study-specific delivery protocol were compared using a t-test in the case of supposed parametric continuous variables, the Wilcoxon rank-sum test in the case of supposed non-parametric continuous variables, and a chi-squared test in the case of categorical variables. Data were analyzed using version Stata/SE V13 (StataCorp LP, College Station, TX). Figure 1 presents the inclusion of the study sample. A total of 1049 women were included in the POPRACT study. Of the study population remaining after exclusion, 809 women had a vaginal delivery. Delivery was documented in the dedicated study protocol for 644 of these women, who thus constituted the present study sample. For the analysis of risk factors in relation to perineal and vaginal tears included 489 and 426 women, respectively, after excluding women with missing data in relevant variables.

Results
Insert Fig. 1 approximately here.
Baseline and obstetric and baseline characteristics of the study sample are shown in Table 1 and Table 2, respectively. The sample had a mean (± SD) age of 28.7 ± 3.7 years (range: 18-41 years), BMI of 24.5 ± 4.4 kg/m 2 (16.4-44.0 kg/m 2 ), gestational age at birth of 40 weeks + 1 day ± 1 week + 3 days (34 weeks + 1 day -42 weeks + 5 days), fetal birth weight of 3513 ± 472 g (1730-5140 g), and fetal head circumference of 34.8 ± 1.5 cm (28.0-38.5 cm). Smoking, symptoms of pelvic organ prolapse during late pregnancy, and lack of manual perineal protection were considered as potential risk factors but were excluded from the analysis of risk factors presented below due to too few exposed women. No statistically significant differences were found between the women whose data were collected according to study-specific delivery protocol registered (n = 644) and those excluded due to missing study protocol (n = 165), except regarding use of epidural analgesia and duration of active second stage of labor. In the excluded group, epidural use was lower (38.2%) and the mean duration of active second stage of labor was longer (48.7 ± 35.4 min; range: 1-189 min).  Missing 1 Obstetric characteristics of the study population. Women where information is missing are not included in the percentage.   Table 4 presents the unadjusted and adjusted odds ratios for second-degree perineal tear and for OASI, respectively. Women with second-degree perineal tear were more likely to be older than 25 years, to have a post-term delivery, to be exposed to oxytocin augmentation, to give birth in the lithotomy position, to have delivery assisted by vacuum extraction, and to have a child heavier than 4000 g or with a head circumference exceeding 35 cm, compared to women who did not have tear or had a tear of first degree (the reference). After adjustment, age above 25 years, vacuum extraction, and fetal weight exceeding 4000 g remained as risk factors significantly increasing the risk of second-degree perineal tear. Women with OASI were more likely to use epidural analgesia, to be exposed to oxytocin augmentation, to have delivery assisted by vacuum extraction, to have a fetal heart beat monitored by scalp electrode, and to have a child heavier than 4000 g or with a head circumference exceeding 35 cm, compared to the reference. After adjustment, vacuum extraction and fetal weight > 4000 g remained as risk factors significantly increasing the risk of OASI. Insert Table 4 approximately here. Table 5 shows the unadjusted and adjusted odds ratios for high vaginal tear. Women with a high vaginal tear were more likely to report heredity of pelvic floor dysfunction and/or connective tissue deficiency, to have induced labor, to deliver a baby whose hand or arm was the presenting fetal part, and to deliver a baby whose head circumference exceeded 35 cm, compared to referent women with no or low vaginal tear. After adjustment, heredity of pelvic floor dysfunction and/or connective tissue deficiency, induced labor, and fetal head circumference > 35 cm remained as risk factors significantly increasing the risk of high vaginal tear. In the adjusted model, augmentation of oxytocin significantly reduced the risk of high vaginal tear despite not being significant in the unadjusted model. Insert Table 5 approximately here.

Odds ratios for high vaginal tear
Odds ratios for the combined effect of delivery mode and fetal weight on the risk of perineal tear Table 6 shows the odds ratios, before and after adjustment, for second-degree perineal tear and OASI, in four different combinations of two risk factors -vacuum extraction and fetal weight: 1) women with spontaneous delivery of a child weighing < 4000 g, 2) women with spontaneous delivery of a child weighing ≥ 4000 g, 3) women with vacuum-assisted delivery of a child weighing < 4000 g, and 4) women with vacuum-assisted delivery of a child weighing ≥ 4000 g. Subgroup 4, in which the two major risk factors were combined, had adjusted ORs for second-degree tear and OASI of 5.4 (95% CI: 1.4, 20.6) and 10.5 (95% CI: 1.4, 80.8), respectively, and the interaction terms for second-degree perineal tear and for OASI were 0.66 (95% CI: 0.14-3.2) and 0.2 (95% CI: 0.02-2.03) respectively, meaning that there was no significant interaction between vacuum extraction and fetal birthweight above 4000 g (data not shown). Table 6 Unadjusted and adjusted odds ratios for the risk of second-degree perineal tear and obstetric anal sphincter injury by delivery mode and fetal weight. Unadjusted and adjusted odds ratios for the risk of second-degree perineal tear and obstetric anal sphincter injury by delivery mode (spontaneous or vacuum extraction) and fetal weight (< 4000 g or ≥ 4000 g). aOR, adjusted odds ratio; CI, confidence interval; OR, odds ratio.
Insert Table 6 approximately here.

Discussion
The incidences of second-degree perineal tear, OASI, and high vaginal tear were 44.7%, 7.3%, and 13.2% respectively. Vacuum extraction and fetal weight above 4000 g were independent risk factors for both seconddegree perineal tear and OASI. Age above 25 years was an independent risk factor for second-degree perineal tear but not for OASI. Heredity of pelvic floor dysfunction and/or connective tissue deficiency, induced labor, and fetal head circumference > 35 cm were independent risk factors for high vaginal tear, whereas oxytocin augmentation appeared to reduce the risk of high vaginal tear.
To our knowledge, this is one of very few observational studies of perineal tears that include tears of second degree. An extensive PubMed search identified only two observational studies reporting the incidence of seconddegree perineal tear [1, 2] and only one of these separately analyzed risk factors for second-degree tears [1] and OASI [17]. As in the present study, Samuelsson et al. found high infant weight to be an independent risk factor for both second-degree tears and OASI, but in their study vacuum extraction was not an independent risk factor for either degree of tear. We did not find that prolonged active phase of second stage of labor led to any increased risk for either OASI or second-degree tears, whereas Samuelsson et al. found that pushing time < 30 min decreased the risk of both [1,17]. An imprecise definition of the active phase of second stage of labor in the present study might partly explain the difference in the results; in the obstetric record system, the midwife entered the time when the woman felt the urge to push, which might not be the same as active pushing. Age above 25 years has been shown to increase the risk of OASI [26], but this is the first study to find an association between age and second-degree tears.
Obstetric risk factors for perineal tears are often interrelated, as is the case for the two largest risk factors identified in thus study: birth weight > 4000 g and vacuum extraction. This was the rationale for the stratification of subgroups according to these risk factors ( Table 6). The odds of OASI in the subgroup with the two major risk factors combined was markedly high; more than tenfold higher than the reference category, even though there was no evidence of positive effect modification and confidence intervals were wide.
The present finding that the incidence of second-degree tears was 44.7% can be contrasted with the incidence of 78.3% in the control group of an interventional study by Edqvist et al. [3]. Since the latter study was also conducted in a Swedish context and published as recently as 2017, explanations other than a true difference in the incidence due to diverging obstetric practice must be sought. Rather, diverging definitions of second-degree tears could explain the difference. Our delivery protocol used the RCOG definitions of perineal tears [24], whereas Edqvist et al. classified vaginal tears with a depth > 0.5 cm as second-degree tears [3].
High vaginal tear was fairly common in our study, affecting 13.2% of women, but to our knowledge no previous studies have specifically evaluated risk factors for high vaginal tears. Vaginal sidewall tears might be an independent risk factor for levator ani avulsion [8], and hence could be a marker for increased future risk of pelvic floor dysfunction. Interestingly enough, we found that heredity of pelvic floor dysfunction and/or connective tissue deficiency was a risk factor for high vaginal tear. One might speculate that a genetic connective tissue deficiency resulting in an increased risk of levator ani avulsion is the link which explains the finding above.
Strengths of this study is the prospective data collection and the assessment of a wide range of risk factors. In the present study we used a validated protocol for documentation of perineal tears, which we have previously shown to deliver more comprehensive information about perineal tears than the most common obstetric record system in Sweden [23].
The sample size of this prospective study (489 and 426 women included in the regression models of perineal tears and vaginal tears respectively) is smaller than in most retrospective studies in the field, which constitutes a limitation of the study. The limited sample size confers a risk of type II errors, and may partly explain why some previously described risk factors did not show the association.

Conclusions
This prospective study showed that fetal weight exceeding 4000 g and vacuum extraction are independent risk factors for both second-degree perineal tears and OASI in primiparous women. Risk factors for high vaginal tears were heredity of pelvic floor dysfunction and/or connective tissue deficiency, induced labor, and fetal head circumference > 35 cm. These tears have been found to be common among primiparous women, and merit more attention.