Risk factor | Reference | Sample size | Adjusted OR or risk ratio (95% CI) | |

Total # | Preterm # (definition) | |||

Age | Cnattingius et al10 | 499 947 | 29 937 (<36 weeks) | OR 2.1 (1.9 to 2.2) for age >/=35 years in nulliparous non-smokers compared with multiparous non-smokers age 20 to 24 years |

OR 2.3 (2.1 to 2.5) for age >/=35 years in nulliparous smokers compared with multiparous nonsmokers age 20 to 24 years | ||||

Meis et al11 | 2929 | 120 (medically indicated <37 weeks) | OR 2.42 (1.57 to 3.74) for age >30 years compared with age </=30 years | |

Premkumar et al12 | 23 425 | 2069 (<37 weeks) | OR 1.29 (1.08 to 1.54) for age <25 years compared with age 30 to 34 years | |

OR 1.27 (1.05 to 1.55) for age >40 years compared with age 30 to 34 years | ||||

Race/ethnicity | Harlow et al13 | 14 948 | 448 (spontaneous <37 weeks) | OR 2.0 (1.4 to 2.9) for black race compared with white |

Meis et al11 | 2929 | 120 (medically indicated <37 weeks) | OR 1.56 (1.02 to 2.40) for black ethnicity compared with white | |

Kistka et al14 | 711 015 | 14 611 (<37 weeks) | OR 2.21 (2.11 to 2.31) for black race compared with white | |

Premkumar et al12 | 23 425 | 2069 (<37 weeks) | OR 1.08 (0.89 to 1.30) for African-American compared with white | |

OR 1.04 (0.88 to 1.22) for Latina/Hispanic compared with white | ||||

OR 0.91 (0.79 to 1.05) for Asian/Pacific Islander compared with white | ||||

Smoking | Harlow et al^{13} | 14 948 | 124 (medically indicated <37 weeks) | OR 1.1 (1.0 to 1.2) for smoking >5 cigarettes/day compared with no smoking |

Bhattacharya et al15 | 7090 | 318 (<37 weeks) | OR 1.47 (1.27 to 1.71) for smoking >10 cigarettes/day compared with no smoking | |

Premkumar et al12 | 23 425 | 2069 (<37 weeks) | OR 1.34 (0.99 to 1.80) for smoking compared with no smoking | |

Hypertension | Meis et al11 | 2929 | 120 (medically indicated <37 weeks) | OR 4.06 (2.29 to 7.22) for chronic hypertension compared with none |

Sibai et al16 | 3499 | 632 (<37 weeks) | OR 2.4 (2.1 to 2.7) for chronic hypertension compared with none | |

Premkumar et al12 | 23 425 | 2069 (<37 weeks) | OR 2.74 (2.28 to 3.29) for chronic hypertension compared with none | |

Diabetes | Harlow et al13 | 14 948 | 124 (medically indicated <37 weeks), 448 (spontaneous <37 weeks) | OR 1.4 (1.1 to 1.8) for abnormal glucose tolerance testing in spontaneous preterm births compared with none |

OR 2.2 (1.2 to 4.1) for abnormal glucose tolerance testing in medically indicated preterm births compared with none | ||||

Sibai et al16 | 3199 | 555 (<37 weeks) | OR 2.7 (2.3 to 3.2) for pregestational insulin-dependent diabetes compared with none | |

Hedderson et al17 | 46 230 | 1956 (spontaneous <37 weeks) | RR 1.23 (1.08 to 1.41) for abnormal glucose tolerance testing compared with none | |

RR 1.42 (1.15 to 1.77) for gestational diabetes compared with none | ||||

Premkumar et al12 | 23 425 | 2069 (<37 weeks) | OR 2.75 (2.18 to 3.47) for pregestational diabetes compared with none | |

Obesity | Hendler et al18 | 2910 | 296 (spontaneous <37 weeks) | OR 0.57 (0.39 to 0.83) for obese (BMI>/=30 kg/m^{2}) compared with non-obese (BMI <30 kg/m^{2}) |

McDonald et al19 | ~1 095 834 | None given (but defined as <37 weeks) | RR 1.24 (1.13 to 1.37) for overweight and obese compared with normal (BMI at least <24 kg/m^{2} for all included studies, with range given meta-analysis) | |

Cnattingius et al20 | 1 599 551 | 67 059 (32–36 weeks) | OR 1.22 (1.18 to 1.27) for BMI 25 to 29.9 kg/m^{2}, medically indicated | |

OR 0.98 (0.96 to 1.01) for BMI 25 to 29.9 kg/m^{2}, spontaneous | ||||

OR 1.62 (1.54 to 1.71) for BMI 30 to 34.9 kg/m^{2}, medically indicated | ||||

OR 1.01 (0.98 to 1.05) for BMI 30 to 34.9 kg/m^{2}, spontaneous | ||||

OR 2.00 (1.84 to 2.18) for BMI 35 to 39.9 kg/m^{2}, medically indicated | ||||

OR 1.06 (1.00 to 1.13) for BMI 35 to 39.9 kg/m^{2}, spontaneous | ||||

OR 2.45 (2.15 to 2.79) for BMI >40 kg/m^{2}, medically indicated | ||||

OR 1.13 (0.95 to 1.33) for BMI >40 kg/m^{2}, spontaneous | ||||

With all groups compared with normal (BMI 18.5 to <25 kg/m^{2}) | ||||

Premkumar et al12 | 23 425 | 2069 (<37 weeks) | OR 1.03 (0.91 to 1.17) for overweight (BMI 25 to 29.9 kg/m^{2}) compared with normal (BMI 18 to 24.9 kg/m^{2}) | |

OR 1.21 (1.04 to 1.40) for obese (BMI >30 kg/m^{2}) compared with normal (BMI 18 to 24.9 kg/m^{2}) | ||||

Hyperlipidaemia | Chatzi et al21 | 625 | 74 (<37 weeks) | RR 1.13 (0.91 to 1.40) for triglycerides per increase in 50 mg/dL |

RR 1.08 (0.88 to 1.33) for HDL per increase in 15 mg/dL | ||||

RR1.17 (0.77 to 1.1.78) for LDL cholesterol per increase in 30 mg/dL | ||||

RR 1.24 (0.99 to 1.56) for total cholesterol increase in 40 mg/dL | ||||

Catov et al22 | 938 | 146 (34–37 weeks) | OR 1.38 (0.80 to 2.36) for fourth quartile total cholesterol (196 to 318 mg/dL) compared with second quartile (156–172) | |

OR 1.86 (1.10 to 3.15) for first quartile total cholesterol (94 to 155 mg/dL) compared with second quartile (156–172) | ||||

OR 1.07 (0.63 to 1.83) for fourth quartile LDL (124 to 233 mg/dL) compared with second quartile (89–104) | ||||

OR 1.60 (0.96 to 2.68) for first quartile LDL (26 to 88 mg/dL) compared with second quartile (89–104) | ||||

OR 0.99 (0.58 to 1.67) for fourth quartile triglycerides (78 to 318 mg/dL) compared with second quartile (44–57) | ||||

OR 1.13 (0.68 to 1.88) for first quartile triglycerides (16 to 43 mg/dL) compared with second quartile (44–57) | ||||

OR 1.52 (0.92 to 2.49) for fourth quartile HDL (64 to 118 mg/dL) compared with second quartile (47–54) | ||||

OR 1.05 (0.62 to 1.79) for first quartile HDL (25 to 46 mg/dL) compared with second quartile (47–54) | ||||

Harville et al23 | 1142 | 67 (<37 weeks) | RR 1.11 (0.84 to 1.46) for total cholesterol as continuous variable | |

RR 0.92 (0.73 to 1.17) for HDL as continuous variable | ||||

RR 1.13 (0.86 to 1.48) for LDL as continuous variable | ||||

RR 1.10 (0.84 to 1.42) for triglycerides as continuous variable | ||||

Magnussen et al24 | 4990 | 272 (<37 weeks) | OR 1.3 (0.9 to 2.0) for fifth quintile total cholesterol (5.7 to 9.9 mmol/L) compared with first quintile (2.1 to 4.1 mmol/L) | |

OR 1.4 (0.9 to 2.2) for first quintile HDL (1.9 to 3.1 mmol/L) compared with first quintile (0.6 to 1.2 mmol/L) | ||||

OR 1.3 (0.8 to 2.2) for fifth quintile triglycerides (1.6 to 9.6 mmol/L) compared with first quintile (0.2 to 0.6 mmol/L) | ||||

Alleman 201340 | 2699 | 200 (<37 weeks) | OR 1.03 (0.89 to 1.19) for total cholesterol as continuous variable | |

OR 1.01 (0.88 to 1.17) for LDL as continuous variable | ||||

OR 0.96 (0.83 to 1.11) for HDL as continuous variable | ||||

OR 1.02 (0.88 to 1.17) for triglycerides as continuous variable |

ORs or RRs with p<0.05 are highlighted in bold.

BMI, body mass index; CVD, cardiovascular disease; HDL, high-density lipoprotein; LDL, low-density lipoprotein.