Table 1 Factors associated with OASI recurrence at second vaginal birth, 2001–2011

Asian

47 (17.0)

910 (20.2)

0.23

0.82 [0.60, 1.13]

0.89 (0.63, 1.26)

Non Asian

226 (81.9)

3594 (79.3)

Reference

Reference

Diabetes

Yes

17 (6.2)

174 (3.8)

0.06

1.64 [0.98, 2.75]

1.76 [1.03, 3.01]

No

259 (39.8)

4358 (96.2)

Reference

Reference

Epidural/spinal/general

Yes

109 (39.5)

1993 (44.0)

0.14

0.82 [0.64, 1.05]

0.79 [0.59, 1.06]

No

167 (60.5)

2539 (56.0)

Reference

Reference

Birth mode

Instrumental

120 (43.5)

2007 (44.3)

0.79

0.97 (0.76, 1.24)

1.04 [0.78, 1.40]

Non Instrumental

156 (56.5)

2525 (55.7)

Reference

Reference

Episiotomy

Yes

121 (43.8)

1939 (42.8)

0.73

1.04 [0.82, 1.33]

1.13 [0.86, 1.50]

No

155 (56.2)

2593 (57.2)

Reference

Reference

Birthweight(kg)

<3.0

29 (10.5)

462 (10.2)

0.34

1.08 [0.70, 1.66]

1.40 [0.89, 2.19]

3.0 - <3.5

95 (34.4)

1636 (36.1)

Reference

Reference

3.5 - <4.0

119 (43.1)

1749 (38.6)

1.17 [0.89, 1.55]

0.94 [0.70, 1.27]

≥ 4.0

33 (12.0)

685 (15.1)

0.83 [0.55, 1.25]

0.53 [0.34, 0.82]

<1

50 (18.1)

1020 (22.5)

0.39

0.76 [0.52, 1.13]

0.78 [0.52, 1.17)

1- < 2

126 (45.7)

1926 (42.5)

1.02 [0.74, 1.40]

1.04 [0.75, 1.45)

2- < 3

59 (21.4)

919 (20.3)

Reference

Reference

≥3

41 (14.9)

666 (14.7)

0.96 [0.64, 1.45]

1.01 [0.66, 1.54]

Mat age

<25

29 (10.5)

550 (12.1)

0.61

0.87 [0.58, 1.30]

0.93 [0.61, 1.41]

25-34

189 (68.5)

3110 (68.6)

Reference

Reference

≥35

58 (21.0)

872 (19.2)

1.09 [0.81, 1.48]

1.07 [0.78, 1.46]

Hypertension

Yes

16 (5.8)

161 (3.5)

0.05

1.67 [0.99, 2.84]

1.56 [0.90, 2.71]

No

260 (94.2)

4371 (96.5)

Reference

Reference

Gestation (weeks)*

<37

4 (1.5)

155 (3.4)

<0.01

0.36 [0.13, 0.99]

0.73 [0.24, 2.19]

37-38

29 (10.5)

840 (18.5)

0.49 [0.33, 0.73]

0.55 [0.36, 0.84]

39-40

197 (71.4)

2779 (61.3)

Reference

Reference

≥41

46 (16.7)

757 (16.7)

0.86 [0.62, 1.19]

0.72 [0.51, 1.02]

Induction/Augmentation

Yes

143 (51.8)

2428 (53.6)

0.57

0.93 [0.73,1.19]

0.95 [0.73, 1.24]

No

133 (48.2)

2104 (46.3)

Reference

Reference

Epidural

Yes

48 (17.4)

892 (19.7)

0.35

0.86 [0.62, 1.18]

0.95 [0.66, 1.38]

No

228 (82.6)

3640 (80.3)

Reference

Reference

Birth mode

Instrumental

24 (8.7)

289 (6.4)

0.13

1.40 (0.91, 2.16)

1.30 [0.80, 2.12]

Non-instrumental

252 (91.3)

4243 (93.6)

Reference

Reference

Episiotomy

Yes

68 (24.6)

1180 (26.0)

0.61

0.93 [0.70, 1.23]

0.84 [0.62, 1.14]

No

208 (75.4)

3352 (74.0)

Reference

Reference

Infant sex*

Male

158 (57.3)

2323 (51.3)

0.05

1.27 [1.00, 1.63]

1.12 [0.87, 1.44]

Female

118 (42.7)

2208 (48.7)

Reference

Reference

Birthweight(kg)*

<3.0

14 (5.1)

543 (12.0)

<0.01

0.54 [0.30, 0.96]

0.56 [0.23, 1.06]

3.0 - <3.5

80 (29.0)

1675 (37.0)

Reference

Reference

3.5 - <4.0

107 (38.8)

1653 (36.5)

1.36 [1.01, 1.83]

1.48 [1.08, 2.02]

≥4.0

75 (27.2)

658 (14.5)

2.39 [1.72, 3.31]

2.89 [1.98, 4.22]

Year of second birth

01-05

68 (24.6)

1098 (24.2)

0.24

Reference

Reference

06-08

87 (31.5)

1643 (36.3)

0.86 [0.62, 1.19]

0.84 [0.60, 1.17]

09-11

121 (43.8)

1791 (39.5)

1.09 [0.80, 1.48]

1.07 [0.78, 1.47]