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Table 4 The odds of all preterm births (20- < 37 weeks gestation) and third trimester preterm births (28- < 37 weeks) across maternal smoking status categories (alternate reference)

From: A gestational vulnerability window for smoking exposure and the increased risk of preterm birth: how timing and intensity of maternal smoking matter

Preterm 20- < 37 weeks

Smoking Status

Crude OR

95% CI

Adjusted ORa

95% CI

 No trimester smokers

0.56

0.53, 0.59*

0.60

0.57, 0.63*

 1st trimester only, low

0.65

0.61, 0.70*

0.66

0.62, 0.71*

 1st trimester only, high

0.64

0.57, 0.71*

0.65

0.58, 0.73*

 1st & 2nd trimesters, low (ref.)

1

1

1

1

 1st & 2nd trimesters,high

1.22

1.09, 1.37*

1.23

1.09, 1.38*

 2nd & 3rd trimesters, low

0.64

0.58, 0.71*

0.63

0.57, 0.70*

 2nd & 3rd trimesters, high

0.70

0.53, 0.92*

0.68

0.51, 0.90*

 All 3 trimesters, low

0.79

0.75, 0.84*

0.79

0.75, 0.84*

 All 3 trimesters, high

0.86

0.81, 0.91*

0.88

0.83, 0.94*

Preterm 28- < 37 weeks

Smoking Status

Crude OR

95% CI

Adjusted ORa

95% CI

 No trimester smokers

0.66

0.62, 0.70*

0.70

0.66, 0.75*

 1st trimester only, low

0.75

0.70, 0.81*

0.76

0.71, 0.82*

 1st trimester only, high

0.72

0.64, 0.82*

0.73

0.65, 0.83*

 1st & 2nd trimesters, low (ref.)

1

1

1

1

 1st & 2nd trimesters, high

1.22

1.07, 1.38*

1.22

1.08, 1.39*

 2nd & 3rd trimesters, low

0.77

0.69, 0.86*

0.76

0.68, 0.85*

 2nd & 3rd trimesters, high

0.82

0.61, 1.09

0.80

0.59, 1.07

 All 3 trimesters, low

0.95

0.89, 1.01

0.95

0.89, 1.01

 All 3 trimesters, high

1.04

0.98, 1.12

1.07

0.99, 1.14

  1. aAnalyses were adjusted for: maternal race/ethnicity, age, education, and marital status; low intensity: 1–19 cigarettes/day; high intensity: ≥ 20 cigarettes/day; *p < 0.05; CI-confidence interval