Results
During this 13-year period, 12,883 women gave birth at the study hospital, and 11,374 of them had a vaginal delivery. The mean incidence of OASI was 2.9% but increased gradually from 2.4% in 1990 to a maximum of 5.9% in 2002. Following vaginal delivery, 421 women (3.7%) were diagnosed with first time 3rd (n=324) or 4th degree (n=97) OASI and included in the study along with 421 matched controls without OASI. Of the 842 women, 550 (65.3%) were primiparous and 292 (34.7%) multiparous. The median parity of the multiparous women was the second child, and the highest parity was six.
Of the non-modifiable variables, we found that maternal age, infant weight, length, head circumference and gestational age, and duration of both the first and second stage of labour were significantly higher for both the primi- and multiparous women with OASI compared to their respective controls (Table 1). Of the potentially modifiable variables related to the delivery, we found that frequencies of episiotomy, amniotomy, augmentation with oxytocin, supine or sitting birth position, instrumental delivery with vacuum extraction and forceps were significantly higher for both the primi- and multiparous women with OASIS compared to their controls (Table 2). Induction of labour occurred more often in the primiparous women with OASI (Table 2).
In the final multivariate conditional logistic regression model, higher maternal age and birthweight for primiparous women and birthweight for the multiparous women were the only non-modifiable variables associated with rates of OASI (Tables 3 and 4). Of the modifiable variables, amniotomy was strongly associated with OASI, both in the primiparous (OR 4.84, 95% CI 2.60–9,02) and multiparous (OR 3.76, 95% CI 1.45–9.76) women, as was augmentation with oxytocin (OR 1.63, 95% CI 1.08–2.46 and 3.70, 95% CI 1.79–7.67, respectively, Tables 3 and 4). Instrumental delivery was associated with OASI in the primiparous women (Table 3) and episiotomy with OASI in the multiparous women (Table 4). We found no significant interactions or multicollinearities.