Among the 746 mothers, 232 (31%) had ever smoked, of whom 161 (22%) quit before pregnancy and 71 (10%) smoked during early pregnancy. Only 13 of the 71 early pregnancy smokers reported smoking beyond the first trimester. The majority of early pregnancy smokers reported smoking 1 to 4 (37%) or 5 to 14 (34%) cigarettes per day. Ever smokers reported a mean of 4 pack-years (range, <1 to 30 pack-years), and early pregnancy smokers reported a mean of 5 pack-years (range, <1 to 25 pack-years).
Mothers who smoked in early pregnancy were younger, less educated, less likely to be married, and had lower household income compared with never smokers (). Smoking mothers were more overweight and gained more weight during pregnancy. Children of mothers who smoked in early pregnancy were not more likely to be born preterm or small for gestational age, but they were breast fed for a shorter duration and had earlier introduction of solid foods. Compared with never smokers, mothers who had previously smoked but quit before the 3 months before conception were older and more likely to be white, parous, and have higher income, but did not differ in BMI or gestational weight gain (). Child television viewing, fast food intake, and sugar-sweetened beverages did not differ by maternal smoking status.
One-half (50%) of the children were boys, and 204 (27%) children were overweight (BMI > 85th percentile) at 3 years. Compared with children of never smokers, children of early pregnancy smokers had higher mean BMI and sum of SS + TR skinfolds and higher prevalence of obesity but did not have different SS:TR (). Mean SBP was slightly higher among children of early pregnancy and former smokers than children of never smokers ().
On multivariable analysis, maternal early pregnancy smoking was strongly associated with overweight. Compared with children of never smokers, among those exposed to smoking in early pregnancy, the adjusted odds ratio (OR) for overweight was 2.2 [95% confidence interval (CI), 1.2, 3.9]. In contrast, children of mothers who had quit smoking before pregnancy were not more overweight (multivariable OR, 1.0; 95% CI, 0.7, 1.6) compared with children of never smokers.
Maternal smoking in early pregnancy was also directly associated with the continuous outcomes BMI z-score (; ) and the sum of SS + TR skinfolds. These associations were somewhat attenuated after adjustment for potential confounders (). It seemed that maternal weight characteristics accounted for most of this attenuation: for example, after adjustment for maternal prepregnancy BMI and gestational weight gain, the effect estimate for early pregnancy smoking on BMI z-score went from 0.43 (95% CI, 0.18, 0.68) to 0.30 (95% CI, 0.06, 0.55) but did not change appreciably further after adjustment for maternal race/ethnicity, education, age, paternal BMI, and fetal growth (0.30; 95% CI, 0.05, 0.55). Mothers who smoked in early pregnancy had children who were heavier at 3 years (0.7 kg; 95% CI, 0.2, 1.2) but not different in height (0.3 cm; 95% CI, −0.7, 1.2), suggesting that the influence of smoking on BMI was mediated by alterations in weight alone. Early pregnancy smoking was not associated with the ratio of SS:TR skinfolds ().
Child SBP and BMI z-score at 3 years of age, comparing children whose mothers smoked in early pregnancy and whose mothers previously smoked but quit before pregnancy with children of never smokers.
Associations of maternal prenatal smoking with child BMI z-score, SS + TR, SS:TR, and SBP at age 3 years
We next studied factors that might serve as intermediates in the pathway between maternal early pregnancy smoking and child adiposity. Inclusion into the multivariable model of glucose tolerance, breastfeeding, early introduction of solid foods, and child behaviors at age 3 (television viewing, consumption of sugar-sweetened beverages and fast food) did not alter observed associations of early pregnancy smoking with BMI z-score (effect estimate adjusted for all factors listed previously, 0.30; 95% CI, 0.04, 0.56). We measured weight and length on 294 participants at 6 months of age. On an analysis limited to this subset of participants, inclusion of change in weight for length z-score from birth to 6 months in the multivariable analysis also did not markedly change the effect estimate for early pregnancy smoking on BMI z-score at 3 years (data not shown).
We observed evidence of a dose effect of smoking on 3-year adiposity. Among children of early pregnancy smokers, BMI z-score was 0.57 units lower among those who had smoked <1 pack-year in their lifetime compared with those who had smoked 10 or more pack-years (p = 0.02). Among former smokers, the number of pack-years smoked was not associated with age 3 BMI z-score. We saw no association of the number of daily cigarettes smoked during early pregnancy with 3-year adiposity, perhaps because of the limited range of the exposure (83% of participants smoked <10 cigarettes daily). Early pregnancy smokers were more likely to report exposure to environmental tobacco smoke in early pregnancy (42% compared with 13% for both former and never smokers). We saw no evidence for an independent effect of environmental tobacco smoke on child adiposity or blood pressure (data not shown).
The small number of women who continued smoking past the first trimester did not allow multivariable analysis of exposure to maternal smoking throughout pregnancy. However, as anticipated, both the prevalence of small for gestational age (14%) and mean unadjusted offspring age 3 BMI z-score (0.83 units) were both higher among these 13 participants than among never smokers (5% and 0.41 units, respectively). Excluding these women, the effect of early pregnancy smoking on child BMI z-score at 3 years remained (0.27 units; 95% CI, 0.003, 0.54).
We obtained SBP measurements from 689 children. After adjustment for blood pressure measurement conditions, maternal sociodemographics, mean third trimester SBP, fetal growth, gestation length, and child sex, age, and height, SBP was 2.4 mm Hg (95% CI, −0.01, 4.9) higher among children of smokers than among children of never smokers (; ). After including child BMI, the effect of smoking was reduced to 1.5 mm Hg (95% CI, −1.0, 3.9), suggesting that higher adiposity among offspring of smokers was partially but not entirely responsible for the elevation in blood pressure.
Smoking before pregnancy was not associated with overall (BMI z-score, SS + TR) or central (SS:TR) adiposity (). Among children of former smokers, estimates of age 3 SBP appeared higher than among children of never smokers (1.5 mm Hg; 95% CI, −0.1, 3.2; ), an effect minimally changed after adjustment for child BMI (1.4 mm Hg; 95% CI, −0.2, 3.0; ).