Table 3. Change in maternal smoking habits during pregnancy and risks of birth complications and childhood overweight.

Maternal smoking		Preterm birth		Small size for gestational age at birth		Childhood overweight
		Odds ratio (95% confidence interval)		Odds ratio (95% confidence interval)		Odds ratio (95% confidence interval)
No maternal smoking in first trimester
Third trimester no smoking		Reference		Reference		Reference
		ncases/total = 4,527/100,634		ncases/total = 8,698/103,740		ncases/total = 11,177/59,070
Third trimester ≤4 cigarettes/day		0.73 (0.40–1.34)		1.20 (0.81–1.78)		1.31 (0.90–1.92)
		ncases/total = 11/278		ncases/total = 28/274		ncases/total = 41/147
Third trimester 5–9 cigarettes/day		1.07 (0.48–2.48)		2.02 (1.18–3.46)*		1.27 (0.64–2.37)
		ncases/total = 6/104		ncases/total = 16/103		ncases/total = 13/51
Third trimester ≥10 cigarettes/day		1.51 (0.65–3.49)		1.74 (0.91–3.32)		1.60 (0.72–3.55)
		ncases/total = 6/80		ncases/total = 11/79		ncases/total = 10/31
Maternal smoking in first trimester ≤4 cigarettes/day
Third trimester quit		0.96 (0.69–1.35)		1.04 (0.82–1.31)		1.20 (0.94–1.53)
		ncases/total = 38/862		ncases/total = 84/859		ncases/total = 98/388
Third trimester ≤4 cigarettes/day		1.05 (0.86–1.27)		1.54 (1.37–1.74)**		1.32 (1.14–1.52)**
		ncases/total = 114/2,261		ncases/total = 328/2,457		ncases/total = 289/1,169
Third trimester 5–9 cigarettes/day		1.15 (0.85–1.55)		2.43 (2.05–2.89)**		1.81 (1.45–2.25)**
		ncases/total = 47/885		ncases/total = 170/880		ncases/total = 121/440
Third trimester ≥10 cigarettes/day		1.37 (0.76–2.47)		2.47 (1.71–3.58)**		1.31 (0.79–2.19)
		ncases/total = 12/186		ncases/total = 36/185		ncases/total = 21/86
Maternal smoking in first trimester 5–9 cigarettes/day
Third trimester quit		1.04 (0.62–1.73)		0.95 (0.64–1.42)		1.32 (0.91–1.92)
		ncases/total = 16/304		ncases/total = 27/304		ncases/total = 41/165
Third trimester ≤4 cigarettes/day		0.86 (0.58–1.28)		1.89 (1.52–2.34)**		1.53 (1.17–2.00)*
		ncases/total = 27/657		ncases/total = 102/654		ncases/total = 80/307
Third trimester 5–9 cigarettes/day		1.00 (0.85–1.18)		2.21 (2.02–2.42)**		1.43 (1.26–1.61)**
		ncases/total = 163/3,551		ncases/total = 630/3,617		ncases/total = 403/1,704
Third trimester ≥10 cigarettes/day		0.99 (0.76–1.30)		2.70 (2.35–3.10)**		1.40 (1.15–1.69)*
		ncases/total = 59/1,330		ncases/total = 265/1,319		ncases/total = 149/632
Maternal smoking in first trimester ≥10 cigarettes/day
Third trimester quit		0.82 (0.46–1.43)		1.06 (0.71–1.57)		1.34 (0.96–1.88)
		ncases/total = 13/285		ncases/total = 28/283		ncases/total = 52/194
Third trimester ≤4 cigarettes/day		1.26 (0.82–1.95)		1.93 (1.46–2.57)**		1.14 (0.81–1.61)
		ncases/total = 22/358		ncases/total = 59/354		ncases/total = 48/192
Third trimester 5–9 cigarettes/day		1.26 (0.97–1.63)		2.79 (2.39–3.25)**		1.46 (1.18–1.80)**
		ncases/total = 62/1,078		ncases/total = 224/1,072		ncases/total = 128/503
Third trimester ≥10 cigarettes/day		1.16 (1.04–1.31)*		2.95 (2.75–3.15)**		1.67 (1.53–1.83)**
		ncases/total = 364/6,949		ncases/total = 1,434/6,940		ncases/total = 849/2,976

Values are odds ratios (95% confidence intervals) from multilevel binary logistic mixed-effects models that reflect the risk of preterm birth, small size for gestational age, and childhood overweight per change in smoking group compared with that of the reference group (nonsmoking in first and third trimester). Preterm birth is defined as birth before the gestational age of 37 weeks. Small size for gestational age is defined as the lowest 10% of sex- and gestational age–adjusted birth weight standard deviation score per cohort. Childhood overweight is overweight and obesity together according to the World Health Organization criteria. Models are adjusted for maternal age, educational level, parity, prepregnancy body mass index, alcohol consumption during pregnancy, and paternal smoking.

*P value < 0.05.

**P value < 0.001.