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Table 2 Risk of OASI in first, second and third live, singleton, vaginal births at term

From: Risk of obstetric anal sphincter injury increases with maternal age irrespective of parity: a population-based register study

Variables   1st births1    2nd births2    3rd births2  
Model 1 Model 2 Model 3 Model 1 Model 2 Model 3 Model 1 Model 2 Model 3
n = 436,670 (n = 412,442) (n = 412,442) n = 379,787 (n = 350,986) (n = 350,982) (n = 140,102) (n = 128,668) (n = 128,668)
Adj OR (95% CI) Adj OR (95% CI) Adj OR (95% CI) Adj OR (95% CI) Adj OR (95% CI) Adj OR (95% CI) Adj OR (95% CI) Adj OR (95% CI) Adj OR (95% CI)
Maternal age (years)
  < 25 1 1 1 1 1 1 1 1 1
 25–29 1.75 (1.69–1.82) 1.66 (1.59–1.72) 1.56 (1.50–1.62) 2.05 (1.83–2.29) 1.78 (1.58–2.01) 1.73 (1.53–1.95) 1.29 (0.83–1.99) 1.09 (0.67–1.77) 1.07 (0.66–1.73)
 30–34 2.05 (1.98–2.13) 1.94 (1.86–2.02) 1.74 (1.67–1.81) 2.84 (2.55–3.16) 2.30 (2.04–2.59) 2.18 (1.93–2.46) 1.90 (1.25–2.89) 1.60 (1.00–2.56) 1.51 (0.94–2.42)
  ≥ 35 2.01 (1.92–2.11) 1.95 (1.85–2.05) 1.64 (1.56–1.73) 3.18 (2.84–3.55) 2.54 (2.24–2.87) 2.32 (2.05–2.63) 2.64 (1.74–4.01) 2.16 (1.34–3.46) 1.97 (1.23–3.16)
Year of delivery
 1999–2004 1 1 1 1 1 1 1 1 1
 2005–2011 0.88 (0.86–0.90) 0.89 (0.87–0.91) 0.88 (0.86–0.90) 0.88 (0.84–0.91) 0.85 (0.82–0.89) 0.85 (0.81–0.89) 0.77 (0.68–0.86) 0.79 (0.70–0.89) 0.80 (0.71–0.90)
Education
 Elementary: 9 ≤ years   0.83 (0.78–0.87) 0.84 (0.79–0.89)   0.92 (0.83–1.02) 0.92 (0.84–1.02)   1.02 (0.83–1.26) 1.01 (0.82–1.25)
 High school   1 1   1 1   1 1
 College or university   1.08 (1.05–1.12) 1.10 (1.07–1.13)   1.01 (0.96–1.06) 1.03 (0.98–1.08)   0.97 (0.85–1.11) 0.97 (0.85–1.11)
Country/Region of birth
 Sweden   1 1   1 1   1 1
 Asia   1.66 (1.58–1.74) 1.58 (1.50–1.65)   1.40 (1.28–1.54) 1.36 (1.24–1.49)   1.43 (1.15–1.78) 1.40 (1.13–1.74)
 Africa   2.01 (1.85–2.20) 1.96 (1.79–2.14)   1.76 (1.50–2.06) 1.71 (1.45–2.00)   2.14 (1.58–2.88) 2.05 (1.52–2.77)
 Other   0.93 (0.89–0.97) 0.93 (0.88–0.97)   0.95 (0.87–1.03) 0.95 (0.87–1.04)   0.92 (0.73–1.17) 0.94 (0.74–1.18)
Smoking in early pregnancy
 No   1 1   1 1   1 1
 Yes   0.81 (0.76–0.86) 0.81 (0.76–0.86)   0.88 (0.78–0.98) 0.87 (0.77–0.97)   0.96 (0.75–1.22) 0.93 (0.73–1.18)
BMI
 Low: <18.5   1.23 (1.14–1.33) 1.22 (1.13–1.31)   1.28 (1.08–1.52) 1.28 (1.08–1.51)   0.71 (0.35–1.43) 0.71 (0.35–1.44)
 Normal weight: 18.5–24.9 1 1   1 1   1 1
 Overweight: 25–29.9   0.96 (0.93–0.99) 0.95 (0.92–0.99)   1.00 (0.94–1.05) 0.99 (0.94–1.05)   0.92 (0.80–1.06) 0.92 (0.80–1.06)
 Obese: ≥ 30   0.90 (0.85–0.94) 0.90 (0.86–0.95)   0.96 (0.89–1.03) 0.96 (0.89–1.04)   0.79 (0.65–0.97) 0.79 (0.64–0.97)
Infant birthweight (g)
  < 3000   1 1   1 1   1 1
 3000–3499   1.70 (1.61–1.81) 1.69 (1.60–1.80)   1.45 (1.25–1.67) 1.44 (1.25–1.67)   1.02 (0.68–1.53) 1.02 (0.68–1.54)
 3500–3999   2.98 (2.81–3.16) 2.88 (2.72–3.05)   2.53 (2.20–2.91) 2.50 (2.17–2.87)   2.40 (1.64–3.51) 2.38 (1.62–3.48)
 4000–4499   5.08 (4.78–5.40) 4.69 (4.41–4.98)   4.25 (3.69–4.90) 4.09 (3.55–4.72)   3.97 (2.70–5.83) 3.84 (2.61–5.65)
  ≥ 4500   8.51 (7.86–9.20) 7.62 (7.04–8.25)   7.39 (6.34–8.61) 7.01 (6.01–8.17)   8.17 (5.48–12.16) 7.70 (5.16–11.49)
Presentation
 Occiput anterior   1 1   1 1   1 1
 Occiput posterior   1.89 (1.78–2.00) 1.43 (1.35–1.51)   1.92 (1.74–2.12) 1.59 (1.44–1.76)   2.00 (1.55–2.59) 1.58 (1.22–2.06)
 Breech   0.87 (0.76–0.99) 1.06 (0.93–1.22)   1.20 (0.72–2.02) 1.26 (0.75–2.12)   1.34 (0.33–5.40) 1.34 (0.33–5.42)
 Other   1.11 (1.04–1.18) 1.03 (0.97–1.11)   1.24 (1.09–1.41) 1.22 (1.07–1.38)   1.72 (1.30–2.28) 1.70 (1.29–2.25)
History of OASI in 1st pregnancy3
 No      1 1    
 Yes      4.38 (4.08–4.70) 4.38 (4.09–4.70)    
History of cesarean section 1st pregnancy3
 No      1 1    
 Yes      7.14 (6.77–7.53) 5.46 (5.14–5.80)    
History of OASI in 2nd pregnancy4
 No         1 1
 Yes         8.92 (7.28–10.90) 8.62 (7.01–10.61)
History of cesarean section 2nd pregnancy4
 No         1 1
 Yes         3.29 (2.45–4.41) 2.93 (2.18–3.95)
Diabetes
 No    1    1    1
 Yes    0.98 (0.79–1.21)    1.35 (1.04–1.73)    0.98 (0.48–2.02)
Epidural analgesia
 No    1    1    1
 Yes    0.93 (0.90–0.95)    1.09 (1.03–1.15)    1.10 (0.93–1.29)
Episiotomy
 No    1    1    1
 Yes    0.89 (0.87–0.92)    1.04 (0.96–1.13)    1.34 (1.01–1.77)
Vacuum extraction or forceps
 No    1    1    1
 Yes    2.87 (2.80–2.95)    2.82 (2.63–3.02)    4.07 (3.33–4.99)
  1. Adjusted Odds Ratio (OR) with 95% Confidence Interval (CI) in three different models: all variables included in the respective model presented
  2. 1Multiple imputation of missing values in BMI
  3. 2Multiple imputation of missing values in BMI, OASI in previous birth, and cesarean section in previous birth
  4. 3Only second births
  5. 4Only third births
  6. The following factors included in Model 3 were not statistically significant in any of the three parity groups, and are not shown in the table: pregestational hypertension, gestational diabetes and preeclampsia