Women who gave birth with an intact perineum or a first-degree tear at the first delivery made up the “unexposed” group. Those who experienced a perineal trauma (an episiotomy or a spontaneous perineal tear of the second degree or higher) at the first delivery were considered “exposed.” Exposure was also categorized according to the severity of the trauma: second-degree spontaneous tear, episiotomy without extension, third- or fourth-degree spontaneous tear, and third- or fourth-degree extension of an episiotomy. All episiotomies were median. The dependent variable was defined as the presence of a second-, third- or fourth-degree spontaneous tear at the second delivery (indicated yes or no).
The association between perineal tears at the second delivery and a history of perineal trauma was primarily measured by the relative risk (RR). The precision of the estimate was given by the 95% confidence interval (95% CI). We also performed unconditional logistic regression to evaluate the influence of potential confounders on the association.29 Unadjusted and adjusted odds ratios (ORs) were obtained. Because the outcome was relatively frequent, the OR overestimated the RR. Trends in proportions were tested with the c2 test for trend.30 Comparisons of proportions were based on Pearson c2 tests.
Results
Of the 3769 women who had their first and second deliveries in the same hospital during the study period, we excluded 579 (15.4%) who had a cesarean delivery, 66 who had a multiple pregnancy, and 31 who had a stillbirth. We also excluded 1198 women who underwent an episiotomy at their second delivery. Among the 1895 secondiparous women left in the analysis, 462 (24.4%) had an intact perineum or a first-degree tear at the first delivery, 333 (17.6%) had a spontaneous second-degree (302) or third- or fourth-degree tear (31); and 1100 (58.0%) had an episiotomy without (911) or with (189) a third- or fourth-degree extension. At the second delivery 1196 (63.1%) delivered with an intact perineum or had a first-degree tear, while 699 (36.9%) had a tear of the second (686) or third or fourth degree (13).
Risk factors for spontaneous perineal tears at the second delivery are presented in Table 1. The unadjusted risk increased with maternal age, gestational age at delivery, birth weight, and fetal head circumference (all tests for trend, P≤.003). The risk was also higher in nonvertex than vertex presentations and in vacuum- or forceps-assisted deliveries than in spontaneous vaginal deliveries. Epidural analgesia, shoulder dystocia, and the training, experience (years since graduation), and identity (data not shown) of the birth attendant were not related to the risk of spontaneous tears in secondiparous women.
Among women who previously gave birth with an intact perineum or a first-degree laceration, 13.4% had a spontaneous tear of the second degree or more at the next delivery Table 2. The risk did not differ whether women had a history of intact perineum (13.0%) or of first-degree tear (14.1% [c2=0.1, P=.7]). In contrast, among women previously exposed to perineal trauma (episiotomy; second-, third-, or fourth-degree tear) 44.5% underwent a spontaneous tear of the second degree or higher at the subsequent delivery. Thus, the overall risk of spontaneous tears (second, third, or fourth degree) was 3.3 times higher in women with a history of perineal trauma at the first delivery than in those without (RR=3.3; 95% CI, 2.6-4.2). The risk of perineal tears at the second delivery increased with the severity of the trauma at the first delivery (test for trend, P <.001). The risk was higher in women with a previous episiotomy without extension (44.8%) than in women with a spontaneous second-degree laceration (36.1%). Severe lacerations at the first delivery yielded the highest risk (54.5%), but the risk was similar whether previous severe lacerations were spontaneous (54.8%) or secondary to an extension of episiotomy (54.5%). Among the 220 women who suffered a third- or fourth-degree perineal tear at their first birth, 2 (0.9%) were delivered with such a tear at their second birth, while among the 1675 who did not suffer a third- or fourth-degree perineal tear at their first delivery, 11 (0.7 %) were delivered with such trauma (RR=1.4; 95% CI, 0.3-6.3).
To verify that the risk factors shown in Table 1 did not confound the associations, we carried out an unconditional logistic regression analysis with simultaneous adjustment for maternal age, birth weight, length of gestation, head circumference, fetal presentation, and mode of delivery. This analysis yielded an adjusted OR of 5.3 (95% CI, 3.9-7.1) for the relation of a history of perineal trauma at the first delivery and the risk of perineal tears (second-, third-, or fourth-degree tears) at the second delivery. As the adjusted OR is similar to the unadjusted OR (5.2; 95% CI, 3.9-6.9), this indicates that the risk factors entered into the model did not confound the association. When the severity of perineal trauma at the first delivery was categorized as in Table 2, the regression analysis again suggested there was no confounding (data not shown).