The present study found evidence of maternal-specific effects of smoking during pregnancy on child's growth. Consistent results were observed between the 1993 and 2004 Pelotas cohort studies, showing that even after adjustment for potential confounding factors, offspring of women who smoked during pregnancy had persistent reductions in length/height at repeated follow-ups from birth to 48 months, had shorter leg length at 48 months, and had higher BMI-for-age z scores at follow-ups at 12 months and older than children of non-smokers. The magnitude of maternal smoking effects on child's growth was greater than and significantly different from paternal smoking effects.
Several prenatal and postnatal factors influence childhood height.24–27
Childhood height is the best predictor of adult height, which is associated with adult health and human capital.28
Leg length growth contributes markedly to stature attainment and is highly influenced by environmental factors in early childhood.7
In our study, consistent reductions in offspring height and height components (in particular leg length and leg-to-trunk ratio) were seen among offspring of women who smoked during pregnancy, findings that have also been reported by Leary et al6
at 7.5 years in the Avon Longitudinal Study of Parents and Children. Although there is some evidence in the literature that the effects of maternal smoking during pregnancy on offspring height decrease with greater offspring age,29
this was not observed in the present study. However, follow-up was limited here to 48 months after birth. Therefore, analyses of future follow-ups in the 2004 cohort will enable further exploration of the long-term effects of maternal prenatal smoking on offspring height.
Smoking is a strongly socially patterned behaviour, thus it is possible that the association between maternal prenatal smoking and child height is due to confounding by wider socioeconomic factors. Although associations attenuated after adjustment for possible confounders, persisting associations were still observed. Furthermore, the markedly discordant maternal–paternal associations observed suggests that maternal smoking during pregnancy may have specific intrauterine effects on offspring height and that residual confounding may be a less likely explanation. Indeed, studies have reported embryotoxic effects of nicotine or other toxic pollutants found in cigarette smoke that lead to delayed skeletal growth.30
Our findings support previous work showing that reductions in weight at birth in children born to mothers who smoke during pregnancy are overcome in infancy.9
The combination of weight being recovered and persisting deficits in height could result in greater body mass in children of maternal smokers. In our study higher BMI z scores were seen among the offspring of women who smoked during pregnancy. Our results are in line with previous studies that reported the association between maternal smoking during pregnancy and childhood overweight and obesity,12 16
and several hypotheses have been postulated to explain this relationship.31
The persistence of the association between maternal prenatal smoking and offspring BMI z scores after adjustment for possible confounders, the discordant maternal–paternal associations observed and the consistency of the findings between the 1993 and 2004 Pelotas cohort studies again make confounding a less likely explanation.
We found a specific negative effect of maternal smoking on head circumference at birth. Our results showed that this association is independent of the relationship between premature birth and maternal smoking. However, deficits in head circumference did not persist in infancy, as also shown in other studies.4 8
A major strength of the present study was the method of data collection (prospective information obtained among large unselected populations and comparable timescales) combined with the use of standardised anthropometric measurements performed by trained fieldworkers, high follow-up rates and low missing data (below 5%) for most variables in both cohort studies. However, some methodological difficulties of the study need to be discussed. First, it is possible that different results would have been obtained if all children whose mothers originally enrolled in the 1993 cohort study were included in the analysis instead of only subsamples. However, no differences in maternal smoking during pregnancy or in socioeconomic or maternal characteristics were found between those children who were followed-up in the 1993 cohort and those originally enrolled in the study (data not shown, available on request), providing some reassurance that attrition is unlikely to have biased the results. Second, both maternal and paternal smoking data rely on maternal self-report. Even though the validity of self-reported smoking has been demonstrated in previous studies and would be acceptable for maternal smoking status,32
no validation has been carried out in our study for paternal information. However, we might anticipate similar, if not even less, reporting bias for paternal smoking than for maternal smoking in pregnancy, as there is likely to be more social pressure on women than on men to not report smoking during pregnancy. Third, even though anthropometric measurements were performed by trained fieldworkers following standardised procedures, leg length (calculated as the difference between standing height and sitting height) is likely to have some measurement error which could have decreased the precision of estimated associations. Finally, it would have been interesting to study the association between maternal passive smoking during pregnancy and child growth. However, no data were available on whether mothers were exposed to passive smoking during pregnancy (ie, if their partner or other people smoked near the woman at home or at work during pregnancy).
Our findings further emphasise the importance of advising women on smoking cessation, particularly in low- and middle-income countries like Brazil. Women of reproductive age should be advised to stop smoking, preferably before attempting to become pregnant, in order to protect their offspring from impaired linear growth and obesity in childhood with potential permanent consequences in adult life.