Characteristic/risk factors | Unadjusted OR | Adjusted OR |
---|---|---|
Maternal age (years) | ||
≤19 | 1 | 1 |
20-29 | 0.63 (0.60–0.67) | 1.08 (1.04–1.18)** |
30-39 | 0.62 (0.59–0.65) | 1.37 (1.28–1.46)*** |
≥40 | 0.87 (0.81–0.94) | 1.82 (1.66–1.99)*** |
Primiparity | 2.20 (2.15–2.24) | 2.41 (2.33-2.48)*** |
Gestational age (weeks) | ||
<28 | 7.30 (6.70–7.95) | 6.26 (5.65–6.93)*** |
28 − 31 + 6 | 7.35 (6.85–7.89) | 5.90 (5.43–6.40)*** |
32 − 36 + 6 | 3.17 (3.06–3.28) | 2.74 (2.63–2.84)*** |
37 − 39 + 6 | 1 | 1 |
40 − 41 + 6 | 0.59 (0.58–0.61) | 0.59 (0.58–0.61)*** |
>42 | 0.51 (0.47–0.54) | 0.44 (0.41–0.47)*** |
Pregravid BMIa | ||
≤24.9 | 1.32 (1.24–1.41) | 1.38 (1.28–1.48)*** |
25-29.9 | 1.00 (0.93–1.08) | 1.06 (0.98–1.15) |
≥30 | 1 | 1 |
Smoking status | ||
Non-smoking | 1 | 1 |
Quitted smoking | 1.28 (1.20 − 1.36) | 1.07 (1.00 − 1.15)* |
Smoking | 2.45 (2.39–2.51) | 2.34 (2.28–2.42)*** |
Single marital status | 1.58 (1.51–1.64) | 1.09 (1.04–1.15)*** |
Socioeconomic status (SES) | ||
Upper white-collar workers | 1 | 1 |
Lower white-collar workers | 1.16 (1.11–1.21) | 1.11 (1.06–1.17)*** |
Blue-collar workers | 1.42 (1.35–1.49) | 1.24 (1.17–1.30)*** |
Other | 1.19 (1.14–1.25) | 1.13 (1.07–1.19)*** |
Missing | 1.36 (1.29 − 1.43) | 1.15 (1.08 − 1.21)*** |
Prior pregnancies (number) | 0.88 (0.87–0.89) | 0.98 (0.97–0.99)*** |
Prior terminations (number) | 1.11 (1.08–1.13) | 1.00 (0.97–1.03) |
IVFb | 1.19 (1.07–1.32) | 0.83 (0.74–0.92)*** |
Chorionic villus biopsy | 1.22 (1.11–1.34) | 1.08 (0.97–1.21) |
Amniocentesis | 1.44 (1.37–1.51) | 1.30 (1.23–1.38)*** |
Placenta previa | 1.47 (1.23–1.75) | 0.75 (0.61–0.91)** |
Placental abruption | 3.36 (2.94–3.83) | 1.41 (1.22–1.64)*** |
Major congenital anomaly | 3.19 (3.08–3.32) | 2.51 (2.40–2.62)*** |
Prior Cesarean section | 0.89 (0.86–0.93) | 1.26 (1.21–1.29)*** |
Time periods | ||
1991 − 1996 | 1 | 1 |
1997 − 2001 | 1.05 (1.02 − 1.08) | 1.04 (1.00 − 1.07)* |
2002 − 2006 | 1.07 (1.04 − 1.10) | 1.08 (1.04 − 1.11)*** |
2007 − 2010 | 1.14 (1.11 − 1.18) | 1.17 (1.13 − 1.21)*** |
Place of residence grouped by hospital districts | ||
1 | 1.40 (1.27 − 1.53) | 1.29 (1.17 − 1.42)*** |
2 | 1 | 1 |
3 | 1.29 (1.17 − 1.43) | 1.20 (1.08 − 1.34)*** |
4 | 1.38 (1.29 − 1.47) | 1.21 (1.13 − 1.30)*** |
5 | 1.64 (1.44 − 1.86) | 1.51 (1.32 − 1.73)*** |
6 | 1.45 (1.31 − 1.61) | 1.38 (1.24 − 1.54)*** |
7 | 1.36 (1.25 − 1.48) | 1.18 (1.08 − 1.30)*** |
8 | 1.13 (1.02 − 1.26) | 1.17 (1.05 − 1.32)** |
9 | 1.19 (1.10 − 1.28) | 1.11 (1.02 − 1.21)* |
10 | 1.35 (1.24 − 1.47) | 1.21(1.10 − 1.32)*** |
11 | 1.66 (1.52 − 1.81) | 1.54 (1.40 − 1.70)*** |
12 | 1.31 (1.17 − 1.46) | 1.13 (1.00 − 1.28)* |
13 | 1.14 (1.06 − 1.23) | 1.05 (0.97 − 1.14) |
14 | 1.33 (1.22 − 1.45) | 1.27 (1.16 − 1.40)*** |
15 | 1.31 (1.22 − 1.54) | 1.32 (1.22 − 1.42)*** |
16 | 1.43 (1.32 − 1.54) | 1.34 (1.23 − 1.45)*** |
17 | 1.35 (1.25 − 1.47) | 1.21(1.11 − 1.32)*** |
18 | 1.15 (1.06 − 1.25) | 1.04 (0.96 − 1.14) |
19 | 1.02 (0.93 − 1.11) | 1.00 (0.91 − 1.10) |
20 | 1.15 (1.07 − 1.23) | 0.97 (0.89 − 1.05) |