Children whose mothers smoked during pregnancy are more likely to be overweight than those who did not, according to researchers in Scotland.
A study by Aberdeen University compared the Body Mass Index (BMI) of siblings at five-years-old whose mothers smoked in-between pregnancies.
It found those exposed to smoke in the womb had a higher BMI than the older sibling who had not.
Data was gathered using the Aberdeen Maternity and Neonatal Databank (AMND) which holds information on all births at the maternity hospital since the mid 1950s.
Figures gathered from schools on more than 700 sibling pair's height and weight at five and a half years old was compared with the AMND records.
Dr Steve Turner, who led the research, said: "This study looked at the relationship between maternal smoking and childhood obesity.
"Previous studies have identified a link between the two but saying that one causes the other is problematic because there are lots of other factors that might explain this relationship, for example people from a poor communities are known to smoke more than those in more affluent communities.
"Also, children in those communities tend to be more obese so it may be that the relationship between smoking and obesity is actually explained by socioeconomic status."
The relationship between pregnant mothers who smoke and childhood obesity has been identified in previous studies, but on this occasion it compared the effect of maternal smoking on siblings.
The results, published in the journal Paediatric and Paediatric Epidemiology, found if the mother started smoking between pregnancies, the younger child had increased BMI compared to their older unexposed brother or sister.
The study allowed researchers to ensure the status of the siblings was the same. It concluded any difference between siblings is likely to be explained by the change in smoking.
Dr Turner added: "We predicted that when a mum has two pregnancies and if she starts smoking between those two pregnancies, the younger child who is exposed to smoke is more likely to be obese according to BMI and that is what we found.
"This study adds to the huge body of evidence that maternal smoking in pregnancy is harmful and the harm isn't just limited to the pregnancy itself - it lasts well-beyond the pregnancy.
"To sum, the relationship between maternal smoking and offspring obesity is complex and is partly explained by other factors but this study provides good evidence that strengthens the association between the two."