Table 2 Multivariate conditional logistic regression analysis for obstetric anal sphincter injury among primiparous women

Non-modifiable variables

Birthweight quartiles

550

   < 3300 g

167

Reference

Reference

  3300–3659 g

141

1.22

0.77–1.96

0.90

0.52–1.55

  3660–4039 g

136

2.36

1.47–3.81

2.27

1.31–3.93

   ≥ 4040 g

106

2.42

1.45–4.01

2.13

1.18–3.84

Maternal age category

550

   < 25 years

198

Reference

Reference

  25–30 years

229

1.57

1.07–2.29

1.68

1.08–2.62

   > 30 years

123

1.87

1.17–2.99

2.04

1.17–3.57

Modifiable variables

Amniotomy

81

4.00

2.27–7.04

4.84

2.60–9.02

Augmentation with oxytocin

284

1.98

1.39–2.81

1.63

1.08–2.46

Instrumental delivery

  Vacuum extraction

74

2.76

1.59–4.81

1.91

1.03–3.57

  Forceps

59

2.64

1.42–4.89

2.37

1.14–4.92