Table 2 Relative and absolute risk^a of preterm birth of the second child^b by gestational age of the first child for the study population for mothers who gave birth to their first two singleton children between 1994 and 2016 in New South Wales (N = 517,558 mothers)

Gest. Age (weeks)

5

10

15

20

25

50

75

90

 22–30

4371 (0.8)

16.65 (13.83–19.95)

18.38 (16.27–20.71)

16.88 (15.34–18.54)

10.37 (9.79–10.96)

3.66 (3.14–4.26)

8.42 (7.63–9.28)

12.77 (11.81–13.79)

27.45 (26.15–28.80)

29

32

34

35

36

38

39

40

 31–32

2338 (0.5)

7.59 (5.38–10.38)

11.95 (9.91–14.28)

12.55 (10.92–14.35)

10.27 (9.54–11.03)

1.67 (1.22–2.27)

5.47 (4.62–6.47)

9.50 (8.37–10.75)

27.20 (25.44–29.04)

31

34

35

36

36

38

39

40

 33

2063 (0.4)

6.61 (4.47–9.39)

8.99 (7.17–11.13)

10.38 (8.83–12.11)

10.52 (9.75–11.33)

1.45 (1.02–2.07)

4.12 (3.34–5.07)

7.85 (6.77–9.09)

27.87 (25.98–29.85)

32

34

35

36

36

38

39

40

 34

3569 (0.7)

4.59 (3.21–6.35)

6.42 (5.22–7.81)

7.52 (6.49–8.66)

8.20 (7.65–8.78)

1.01 (0.73–1.39)

2.94 (2.44–3.55)

5.69 (4.97–6.50)

21.71 (20.39–23.10)

33

35

36

36

37

38

39

40

 35

6062 (1.2)

4.35 (3.27–5.68)

4.75 (3.94–5.68)

5.93 (5.21–6.73)

7.06 (6.64–7.49)

0.96 (0.74–1.23)

2.18 (1.84–2.58)

4.49 (3.99–5.04)

18.69 (17.73–19.69)

34

35

36

37

37

38

39

40

 36

12,114 (2.3)

2.55 (1.96–3.28)

3.32 (2.82–3.88)

3.80 (3.37–4.26)

5.35 (5.08–5.64)

0.56 (0.44–0.71)

1.52 (1.32–1.75)

2.87 (2.59–3.19)

14.18 (13.57–14.81)

35

36

37

37

37

38

39

40

 37

26,310 (5.1)

2.39 (1.96–2.89)

2.61 (2.29–2.97)

2.73 (2.47–3.01)

3.70 (3.53–3.87)

0.52 (0.44–0.62)

1.20 (1.07–1.34)

2.06 (1.90–2.24)

9.79 (9.44–10.16)

35

37

37

37

38

39

39

40

 38

68,508 (13.2)

1.31 (1.10–1.56)

1.61 (1.44–1.80)

1.79 (1.64–1.95)

2.29 (2.20–2.39)

0.29 (0.25–0.33)

0.74 (0.68–0.80)

1.35 (1.27–1.44)

6.08 (5.90–6.26)

36

37

38

38

38

39

40

40

 39

115,699 (22.4)

1.20 (1.03–1.40)

1.22 (1.10–1.36)

1.24 (1.14–1.34)

1.46 (1.40–1.52)

0.26 (0.24–0.30)

0.56 (0.52–0.61)

0.94 (0.88–1.00)

3.86 (3.75–3.98)

37

38

38

38

38

39

40

40

 40

168,287 (32.5)

1

1

1

1

0.22 (0.20–0.24)

0.46 (0.43–0.49)

0.76 (0.72–0.80)

2.65 (2.57–2.73)

37

38

38

38

39

39

40

41

 41

96,443 (18.6)

0.94 (0.79–1.12)

0.89 (0.79–1.00)

0.83 (0.75–0.91)

0.78 (0.74–0.82)

0.21 (0.18–0.24)

0.41 (0.37–0.45)

0.63 (0.58–0.68)

2.07 (1.98–2.16)

38

38

38

39

39

40

40

41

 42–44

11,794 (2.3)

1.00 (0.66–1.46)

1.00 (0.75–1.30)

0.87 (0.69–1.09)

0.77 (0.67–0.87)

0.22 (0.15–0.32)

0.46 (0.35–0.60)

0.66 (0.53–0.82)

2.03 (1.79–2.30)

38

38

38

39

39

40

41

41