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Table 4 Adjusted Odds Ratios (OR) of obstetric anal sphincter injuries (OASIS) (n = 1,137/n = 256) among primiparous (n = 109,596) and multiparous women with vaginal delivery (n = 146,064) in Non-University central Hospitals divided into three groups based on OASR rates (Low, Medium, High) between 1997 and 2007 in Finland.

From: Up to seven-fold inter-hospital differences in obstetric anal sphincter injury rates- A birth register-based study in Finland

CHARACTERISTICS/INTERVENTIONS Adjusted OR 95% CI p value
PRIMIPAROUS,
n = 109,596
   
Mode of delivery    
Vaginal spontaneous 1   
Forceps 24.40 6.90-86.29 ≤0.001
Vacuum assistance 4.76 3.70-6.14 ≤0.001
Episiotomy and vaginal spontaneous 1   
Episiotomy and forceps 0.18 0.04-0.75 0.018
Episiotomy and vacuum assistance 0.50 0.38-0.65 ≤0.001
Epidural analgesia 0.79 0.70-0.89 ≤0.001
Maternal age (year)    
≤19 1   
20-29 1.74 1.22-2.49 0.002
30-39 2.23 1.56-3.21 ≤0.001
≥40 1.56 0.78-3.12 0.21
Birth weight (g)    
≤2,999 1   
3,000-3,499 1.91 1.47-2.47 ≤0.001
3,500-3,999 2.74 2.12-3.53 ≤0.001
≥4,000 4.27 3.27-5.59 ≤0.001
Hospitals OASIS rate    
Low: ≤0.5% 1   
Medium: 0.6-1% 1.84 1.38-2.47 ≤0.001
High: 1.1-1.4% 2.97 2.29-3.85 ≤0.001
MULTIPAROUS,
n = 146, 064
   
Mode of delivery    
Vaginal spontaneous 1   
Vacuum assistance 8.33 4.78-14.54 ≤0.001
Episiotomy and vaginal spontaneous 1   
Episiotomy and vacuum assistance 0.41 0.20-0.83 0.01
Episiotomy 2.82 2.12-3.76 ≤0.001
Epidural analgesia 1.38 1.03-1.84 0.03
Birth weight (g)    
≤2,999 1   
3,000-3,499 2.03 0.80-5.21 0.14
3,500-3,999 3.92 1.61-9.76 0.003
≥4,000 7.56 3.15-19.0 ≤0.001
Hospitals OASIS rate    
Low: ≤0.1% 1   
Medium: 0.1-0.2% 4.37 1.62-11.81 0.004
High: 0.3-0.4% 8.24 3.03-22.41 ≤0.001
  1. Odds ratios (OR) adjusted for mode of delivery, hospitals, induction, oxytocin, episiotomy, occiput posterior presentation (OP), epidural analgesia, spinal analgesia, use of nitrous oxide gas, paracervical block, maternal age, body mass index [BMI = body weight in kilograms/height in meters squared], length of active second stage of birth, birth weight, and head circumference. OP, spinal analgesia, BMI, length of active second stage of birth, and head circumference were adjusted for the years 2004-2007. All continuous variables (maternal age, BMI, birth weight, head circumference, and length of active 2nd stage of birth) were classified as categorical variables.