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)

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

^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