We have shown that breastfeeding wasn’t significantly associated with a decrease of passive smoking issues at 9 months for children of smoking mothers, when considering an adjustment on neonatal health issues or and socioeconomic status of mothers. On the contrary, in a secondary analysis, healthy children seemed to be breastfed for longer (more than 18 weeks).
Strengths and limits
Our study was the first to explore the effect of breastfeeding on tobacco’s adverse effects on children of smoking mothers in France. It has been done on public funding and without conflicts of interest.
8th day assessment of children health is a standardised, reproducible and systematic survey asked to mothers for all the new-borns.
9th month assessments of health collecting was estimated to 42%. This inclusion bias can be explained by the differences between doctors in the involvement in preventive medical care. It can be considered as non-differential.
Our sample size has permitted to reach a statistical power of 99.8%. Our study was robust. Groups for breastfeeding duration were statistically homogeneous in number. Missing values proportion was acceptable. Bias were non differential.
This study contained some methodological limits.
First, neither smoking duration nor smoking cessation were analysed. Smoking behaviour during or after pregnancy could have an effect on breastfeeding’s decision or duration. An Italian study on 3669 women, published in 2012, found that smoking mothers who are breastfeeding smoke less than those who don’t breastfeed, and that those who have quit smoking during pregnancy relapse more often until 12 months after the delivery when they don’t breastfeed (16). Another study , on 6093 Norwegian infants, published in 2015, found that breastfeeding duration was shorter when the mother smoked (17). In this study, the choice was to include all smoking mothers’ children without any selection because we didn’t want to select only strongly exposed children and make an interpretation bias. We wish to emphasise that there isn’t any threshold under which there isn’t adverse effects due to tobacco (14).
Second, the data didn’t permit to describe the exposure to tobacco by making a difference between being exposed as a foetus to a smoking pregnant mother and as a child to smoking parents (mother and/or father) through the breastmilk and/or through the contaminated air. Our choice was to consider the children to be or have been exposed to tobacco when the mother declared to have smoked at least one time during her pregnancy. This consideration may have diluted a potential effect of breastfeeding on the children with a high or long exposition to tobacco. To our knowledge, these considerations and their impact on the results haven’t been yet published in any article. This could be justified by the fact that most of the smoking mothers who succeed to quit smoking during pregnancy have a relapse few days or weeks after the delivery (16).
Third, in our study, we haven’t taken account of the type of breastfeeding (exclusive or not). An American study on 393 infants, published in 2014, has shown that in-hospital formula use increases early breastfeeding cessation among first-time mothers intending to exclusively breastfeed (18).
This points should be studied more precisely in further researches.
Finally, our explorations were limited to one geographical department and may not permit to extrapolate to other departments or countries.
Even though tobacco products can diffuse through breast milk, our study showed that breastfed children are not significantly sicker than bottle-fed children. Tobacco isn’t a contraindication for breastfeeding, which is confirmed by evidence in medical care (19).
On the contrary, after adjusting on our confusion variables, we have shown that breastfeeding duration (even if higher than 18 weeks) was no longer associated with a healthy status. We don’t find any dose-response effect between breastfeeding effect and its duration. No causality argument was found.
Observed difference in univariate analysis could be explained by an inversed causality: healthy children were breastfed for longer.
An American study on 1177 mothers, published in 2013, found that early termination was positively associated with mothers' concerns regarding illness or need to take medicine (20). A Turkish study, published in 2011, have found that babies with regurgitations, intensive crying, colic, or a shorter sleeping time have a lower breastfeeding score (21). An Australian study, published in 2012, on 179 infants, separated in 2 groups (92 term healthy infants and 87 sick/preterm infants), haven’t found any link between healthy status and breastfeeding duration, possibly due to lack of statistical power (22).
The better explanation is that children who have adverse effects of passive smoking, like chronic otitis media or chronic lung diseases (bronchiolitis, asthma, etc.), have more feeding difficulties or are admitted in hospital more frequently or for longer (23,24), which doesn’t allow to benefit from optimal conditions for the prolongation of breastfeeding. We can also say that, as it is stated in the literature, smoking induces a lower milk production and a poor quality of breast milk (25). These negative feelings of mothers on their capacity to feed their child is recognized to induce an early weaning in breastfeeding (13). These points have to be studied more precisely in further works.