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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