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. Author manuscript; available in PMC: 2013 May 1.
Published in final edited form as: Am J Perinatol. 2008 May;25(5):259–264. doi: 10.1055/s-2008-1075040

Risk Factors for Birth Canal Lacerations in Primiparous Women

Rafael T Mikolajczyk 1, Jun Zhang 1, James Troendle 1, Linda Chan 2
PMCID: PMC3640278  NIHMSID: NIHMS460315  PMID: 18509884

Abstract

Lacerations of the birth canal are common side effects of vaginal birth. They are potentially preventable. Although serious long-term consequences have been identified for severe perineal lacerations, less attention has been paid to lacerations in other locations and how the risk factors vary for different lacerations. We analyzed a dataset including 1009 primiparous women with singleton pregnancies and vaginal deliveries, and we examined risk factors for third- and fourth-degree perineal lacerations and periurethral, vaginal, and labial lacerations using logistic regression analysis. Large fetal size (≥ 3500 g) substantially increased the risk of perineal (odd ratio [OR], 3.8; 95% confidence interval [CI], 1.8 to 7.9) and periurethral (OR, 2.3; 95% CI, 1.0 to 5.0) lacerations but not other types of lacerations. Episiotomy had no impact on perineal lacerations (OR 0.9) but had very strong protective effects for other lacerations (OR 0.1). Prolonged second stage of labor (>120 minutes) increased the risk of perineal and vaginal lacerations but reduced the risk for periurethral lacerations. Instrumental deliveries were significant risk factors for third- and fourth-degree perineal lacerations, with by far the strongest effect for low forceps (OR 25.0 versus <3 for outlet forceps, outlet vacuum, and low vacuum). We concluded that separating different birth canal lacerations is critical in identifying risk factors and potential preventive strategies.

Keywords: Laceration, perineum, duration of second stage, episiotomy


Laceration can be a serious side effect of vaginal delivery. Whereas lacerations can occur in different locations, the third- and fourth-degree perineal lacerations involving an injury of the anal sphincter have the highest clinical importance and draw most attention because injury to the anal sphincter is linked to short-term and long-term fecal incontinence.14

The third- and fourth-degree degree perineal lacerations were reported to occur in 2 to 19% of vaginal deliveries in the United States, depending on the population and management of delivery.5 Several risk factors have been identified, including primiparity, large fetal size, operative vaginal delivery, occiput posterior position, prolonged second stage of labor, and horizontal position of the mother during delivery.617 Less attention has been paid to other lacerations such as periurethral, vaginal, labial, or cervical lacerations. Because these lacerations may be preventable, we examined the risk factors for all types of lacerations in primiparous women.

METHODS

We used data from an observational retrospective study. Detailed description of the study population has been provided elsewhere.18 Briefly, the study was designed to examine whether dramatic and rapid increase in epidural analgesia use had a significant impact on the rate of cesarean delivery. We systematically selected a sample of 500 to 700 women from two time periods: before (October 1992 to October 1993) and after (October 1995 to March 1996) a change in the policy advocating the use of epidural analgesia for labor in a military hospital in Hawaii. All women who met the following criteria were included: primiparous, singleton pregnancy, mother’s age between 18 and 34 years at admission, gestational age between 37 0/7 and 41 6/7 weeks at delivery (based on last menstrual period or ultrasound), birthweight between 2500 and 4000 g, vertex presentation, and spontaneous onset of labor. Women whose cervical dilatation was >7 cm at admission or duration of labor from admission to delivery was <3 hours were excluded because they would potentially not have sufficient time to use epidural analgesia.

The following information was extracted from medical records by trained midwives: demographic information, assessment at admission, labor progression, labor and delivery summary, anesthesia, and postpartum information. Between the two time periods of data collection, some changes occurred, such as increased use of vacuum versus forceps for operative vaginal delivery, reduction in episiotomy, and increasing maternal age and body mass, most of which reflect changes in practice patterns and social trends but are not necessarily related to the risk factors of interest. Nonetheless, to avoid potential confounding we controlled for the time period in our analysis.

The total sample size of the original study was 1329 women. For the current analysis we restricted the dataset to women who were delivered vaginally (N = 1159). Furthermore, we removed women with missing information for any of the considered variables (13% of eligible women), resulting in a final sample size of 1009 women. Five types of lacerations based on clinical diagnosis were recorded in the medical records: perineal (with additional classification into first to fourth degree), periurethral, vaginal, labial, and cervical lacerations. Because cervical lacerations occurred only in five subjects, we excluded these women from further analysis.

The selection of variables considered in this analysis was based on findings from previous studies. We first present the population characteristics. Women with any types of lacerations were compared with women without laceration in bivariate analysis. All variables associated with the outcome in bivariate analysis at p <0.05 level were included as potential covariates in the initial multinomial logistic regression model. To construct the final model, we used forward selection of variables based on the likelihood ratio test with a significance level of p <0.05. Given the relatively small sample size in relation to the number of variables investigated and the absence of strong theory in support of effect modification, we refrained from the analysis of interactions.

RESULTS

The first column in Table 1 describes the population characteristics. Two thirds of the women were <25 years; 65% were white. Twenty-one percent of the women were overweight or obese before pregnancy. The median weight gain during pregnancy was around 15 kg. The majority of women were primigravid. Half of the women in the sample had epidural analgesia. Episiotomy was performed in two thirds of the sample (midline episiotomy was used in all but five cases). Information about duration of active phase of first stage (from 4 to 10 cm cervix dilation) was missing for 22% of the women (because they were not examined at 4-cm dilation). Seventeen percent of women had a second stage of labor >2 hours. Five percent of deliveries had occiput-posterior or transverse presentation, 74% had spontaneous delivery, and 39% of the women experienced no lacerations.

Table 1.

Incidence of Lacerations by Women’s Characteristics

Variable Population Characteristics Lacerations
Perineal 3rd or 4th Degree Periurethral Vaginal Labial

(N = 182) (N = 88) (N = 94) (N = 64)


N (%) % % % %
Age of the mother
 <20 y 117 (12) 13.7 8.6 11.1 6.8
 20–24 y 549 (54) 16.8 9.5 8.6 6.7
 25–29 y 262 (26) 21.4 8.4 10.3 5.0
 ≥ 30 y 81 (8.0) 22.2 4.9 8.6 7.4
Race of the mother
 Asian 113 (11) 25.7 8.0 10.6 6.2
 Black 121 (12) 12.4 11.6 12.4 6.6
 Hispanic 69 (6.9) 20.3 7.3 8.7 8.7
 White 660 (65) 17.6 8.2 9.1 5.9
 Other 46 (4.6) 17.4* 13.0 2.2 8.7
Mother’s BMI prior to pregnancy
 <20 kg/m2 236 (23) 24.2 6.4 8.1 5.1
 20–24.9 kg/m2 564 (56) 16.8 10.1 9.2 7.1
 25–29.9 kg/m2 159 (16) 13.8 8.2 11.3 6.9
 >30 kg/m2 50 (5.0) 16.0 6.0 10.0 2.0
Weight gain during pregnancy
 <10 kg 135 (14) 15.6 4.4 6.7 10.4
 10–20 kg 631 (63) 17.3 8.7 10.3 5.2
 ≥ 20 kg 243 (24) 21.4 11.1 8.2 7.0
Gestational age at delivery
 37 wk 67 (6.6) 10.5 4.5 10.5 9.0
 38 wk 179 (18) 15.6 8.9 7.8 5.0
 39 wk 290 (29) 16.6 11.0 7.6 7.2
 40 wk 347 (35) 19.3 8.1 11.2 5.5
 41 wk 126 (12) 25.4 7.1 9.5 7.1
Birthweight
 2500–2999 g 160 (16) 8.8 8.1 10.0 8.8
 3000–3499 g 457 (45) 19.0 8.3 8.8 5.5
 3500–4000 g 392 (39) 20.7 9.4 9.7 6.4
Epidural anesthesia
 No 494 (49) 21.7 9.9 9.3 5.3
 Yes 515 (51) 14.6 7.6 9.3 7.4
Episiotomy
 No 382 (38) 5.5 13.6 14.7 13.1
 Yes 627 (62) 25.7 5.7 6.1 2.2
Duration of active phase (cervix dilation 4–10 cm)
 Missing 219 (22) 19.6 10.1 9.1 4.6
 <5 hr 225 (33) 15.8 8.7 11.0 7.2
 ≥ 5 hr 455 (45) 18.9 8.1 8.1* 6.6*
Duration of second stage
 <30 min 243 (24) 11.5 13.2 7.4 9.9
 30–60 min 309 (31) 14.9 11.3 11.7 5.8
 60–90 min 177 (18) 20.9 5.1 6.2 4.0
 90–120 min 112 (11) 21.4 8.0 9.8 2.7
 >120 min 168 (17) 28.0* 1.8* 10.7 7.1
Presentation at delivery
 Occiput anterior 951 (95) 17.5 9.04 9.2 6.4
 Occiput transverse or occiput posterior 51 (5.1) 31.4 0.00 11.8* 5.9
Delivery mode
 Spontaneous 750 (74) 10.7 9.6 8.1 6.5
 Outlet forceps 38 (3.8) 29.0 2.6 7.9 7.9
 Low forceps 99 (9.8) 58.6 5.1 18.2 6.1
 Outlet vacuum 93 (9.2) 24.7 8.6 10.8 6.5
 Low vacuum 29 (2.9) 34.5 6.9 6.9 0.0
*

p <0.05,

p <0.01,

p <0.001, based on chi-square test comparing women with the given subtype of lacerations with women who did not have this laceration.

Table 1 shows that the third- and fourth-degree perineal lacerations tended to be more common in women who were older (not statistically significant), were underweight prior to pregnancy, had a larger fetus, and received no epidural analgesia. Episiotomy, longer duration of the second stage, occiput-posterior or transverse presentation, and delivery with low forceps were associated with a higher incidence of the perineal laceration. In contrast, longer duration of second stage also reduced the risks of periurethral and labial lacerations. Also episiotomy was consistently associated with lower incidence of these lacerations. Low forceps increased the risk of vaginal laceration.

We then included all the variables just described into a multivariable multinomial logistic regression model using no laceration as a reference category and forward variable selection. Table 2 presents the risk factors for types of lacerations from the final model (controlling for all factors in the table as well as the study period). Older maternal age was only associated with third- and fourth-degree perineal laceration. Large fetal size substantially increased risk of third- and fourth-degree perineal and periurethral lacerations. The effect was much smaller on vaginal and labial lacerations. Episiotomy did not affect the risk of perineal lacerations in the multivariable model (in contrast to bivariate findings) but had very strong protective effects on other lacerations. Prolonged second stage increased risk of perineal and vaginal lacerations. Although all instrumental deliveries were risk factors for third- and fourth-degree lacerations, low forceps was the most prominent with a relative risk of 25 (95% CI, 12.2 to 51.2). Low forceps also increased the risk of vaginal lacerations sevenfold.

Table 2.

Variables Associated with Lacerations in Nulliparous Women (Each Laceration Category Compared with No Laceration)

Variable Risk of Lacerations
Perineal (3rd or 4th Degree) Periurethral Vaginal Labial

OR (95% CI) OR (95% CI) OR (95% CI) OR (95% CI)
Age of the mother
 <20 y Reference Reference Reference Reference
 20–24 y 1.0 [0.5, 1.9] 1.0 [0.4, 2.2] 0.6 [0.2, 1.3] 0.8 [0.3, 2.2]
 25–29 y 2.0 [1.0, 4.2] 1.6 [0.7, 4.0] 1.4 [0.6, 3.4] 0.6 [0.2, 2.1]
 ≥ 30 y 2.0 [0.8, 5.0] 0.8 [0.2, 3.5] 0.9 [0.2, 3.1] 2.0 [0.5, 7.5]
Birthweight
 2500–2999 g Reference Reference Reference Reference
 3000–3499 g 3.4 [1.6, 7.0] 1.5 [0.7, 3.3] 1.4 [0.7, 3.2] 0.9 [0.4, 2.0]
 3500–4000 g 3.8 [1.8, 7.9] 2.3 [1.0, 5.0] 1.8 [0.8, 4.1] 1.5 [0.6, 3.0]
Episiotomy
 No Reference Reference Reference Reference
 Yes 0.9 [0.5, 1.6] 0.1 [0.0, 0.1] 0.1 [0.0, 0.1] 0.0 [0.0, 0.1]
Duration of second stage
 <30 min Reference Reference Reference Reference
 30–60 min 1.4 [0.8, 2.6] 0.9 [0.5, 1.7] 2.4 [1.1, 5.5] 0.7 [0.3, 1.6]
 60–90 min 1.9 [1.0, 3.7] 0.3 [0.1, 0.8] 1.0 [0.4, 2.9] 0.4 [0.1, 1.2]
 90–120 min 2.0 [1.0, 4.3] 0.7 [0.3, 1.9] 2.8 [1.0, 7.8] 0.4 [0.1, 1.6]
 >120 min 1.9 [1.0, 3.9] 0.2 [0.1, 0.7] 2.4 [0.9, 6.6] 0.7 [0.3, 2.2]
Delivery mode
 Spontaneous Reference Reference Reference Reference
 Outlet forceps 2.8 [1.2, 6.4] 0.4 [0.0, 3.2] 1.0 [0.2, 3.9] 1.2 [0.3, 5.1]
 Low forceps 25.0 [12.2, 51.2] 2.3 [0.7, 7.7] 7.0 [2.6, 18.7] 1.6 [0.3, 7.9]
 Outlet vacuum 2.8 [1.5, 5.2] 1.7 [0.6, 4.4] 1.8 [0.7, 4.7] 1.4 [0.4, 4.6]
 Low vacuum 2.2 [0.9, 5.6] 1.4 [0.3, 6.9] 1.5 [0.3, 8.0] NA

Note: Multinomial logistic regression adjusted for period of data collection and all other variables in the table.

If a woman had more than one type of laceration, which is uncommon in our study (8%), we assigned her to the most severe laceration.

OR, odds ratio; CI, confidence interval; NA, not available.

DISCUSSION

Our study shows that risk factors for lacerations in different locations of the birth canal vary substantially. Higher maternal age increases the risk of third- and fourth-degree perineal lacerations. Large fetal size brings along with it a higher risk of perineal and periurethral lacerations. Instrumental deliveries, low forceps in particular, are significant risk factors for most lacerations. Episiotomy is not associated with third- and fourth-degree perineal lacerations but exerts strong protection against periurethral, vaginal, and labial lacerations. A long second stage of labor increases risks for perineal and vaginal lacerations but tends to reduce risks for periurethral and labial lacerations.

Most of our findings are consistent with previous literature. For example, instrumental deliveries are consistently associated with a higher risk of lacerations in the literature, and vacuum extraction results in fewer lacerations than forceps.1922 We found that this was the case mainly for perineal lacerations and, to a lesser extent, for vaginal, periurethral, or labial lacerations. Our study further showed that the disadvantage of forceps as compared with vacuum was restricted to low forceps, whereas outlet forceps presented a similar risk to vacuum.

In our bivariate analysis, episiotomy was strongly associated with third- and fourth-degree perineal laceration (25.7% with episiotomy versus 5.5% without), which is consistent with previous studies.2325 However, after controlling for other factors, especially instrumental delivery, in the multivariable model, episiotomy was no longer a risk factor, suggesting that most of the risk associated with the perineal laceration was mediated through instrumental delivery or due to confounding by indication. However, our study shows that episiotomy is associated with a reduced risk for lacerations other than perineal. The latter finding was previously reported from randomized trials of episiotomy26 and observational studies.27,28 In a meta-analysis of four randomized trials comparing restrictive versus routine use of episiotomy, the restrictive use of episiotomy was associated with a risk increase of 1.79 (95% CI, 1.55 to 2.07) for anterior perineal lacerations.26

We also found that duration of the second stage of labor may have opposing effects on different lacerations. Longer duration was associated with a higher risk of perineal and vaginal lacerations, consistent with previous studies.5,7,10,16,29 On the contrary, a shorter second stage appears to be associated with a higher risk of periurethral and labial lacerations. To our best knowledge, the latter findings have not been reported previously. This association might be explained by mechanics of the delivery: a fast extension of fetal head in a short second stage resulting in the anterior trauma.

Several limitations in our study should be noted. First, like most previous studies, the diagnosis of lacerations was based on clinical judgment in our study. Comparing with a randomized trial in which assessment of lacerations was standardized,30 our study appeared to have a lower incidence of most lacerations (i.e., superficial tears may not have been recorded). Thus our findings may be more applicable to more substantial lacerations. Second, classification of the degree of perineal laceration could vary from physician to physician. This variation is more likely to be non-differential, which could have drawn the results toward the null.

Third, because the data were collected a decade ago, obstetric practice may have changed to certain degree. For instance, episiotomy is less used now. Although differences in obstetric practice could potentially affect the incidence of lacerations, it is less likely that the practice change would materially influence the associations between the risk factors and the different types of lacerations in the multivariable model controlling for other factors. Finally, our study population consisted of women of 18 to 34 years of age with a normal birthweight baby. Thus our findings may not be extrapolated to women outside this age range or with macrosomia or low birthweight babies.

In conclusion, we confirmed the association between longer second stage and perineal lacerations, but our findings point toward an increased risk of periurethral or labial lacerations in deliveries with a short second stage. Future studies should classify subtypes of laceration according to their location to allow more specific analyses. Findings from detailed analyses may help identify preventive strategies.

Acknowledgments

This study was supported by the Intramural Research Program at the National Institute of Child Health and Human Development, the National Institutes of Health. The views expressed in this article are those of the authors and do not reflect the official policy or position of the Department of the Navy, Department of Defense, or the U.S. government.

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