Table 3 Incidences preterm birth stratified by singletons and multiples

Singletons

Total PTB < 32 weeks

162

6.1

691

6.5

0.94

0.79–1.12

Spontaneous PTB < 32 weeks

107

4.0

380

3.6

1.13

0.91–1.40

Iatrogenic PTB < 32 weeks

55

2.1

311

2.9

0.71

0.53–0.95

Total PTB < 28 weeks

58

2.2

232

2.2

1.01

0.75–1.34

Spontaneous PTB < 28 weeks

46

1.7

141

1.3

1.31

0.94–1.83

Iatrogenic PTB < 28 weeks

12

0.5

91

0.9

0.53

0.29–0.97

Multiples

N = 420

N = 1713

Total PTB < 32 weeks

38

90.5

131

76.5

1.20

0.82–1.75

Spontaneous PTB < 32 weeks

34

81.0

92

53.7

1.55

1.03–2.36

Iatrogenic PTB < 32 weeks

4

9.5

39

22.8

0.41

0.15–1.16

Total PTB < 28 weeks

18

42.9

31

18.1

2.43

1.35–4.39

Spontaneous PTB < 28 weeks

17

40.5

24

14.0

2.97

1.58–5.58

Iatrogenic PTB < 28 weeks

1

2.4

7

4.1

0.58

0.07–4.74