The negative effects of smoking during pregnancy are well established, and it seems the influence can be transferred on to the grandchildren and great-grandchildren of smokers - even if the intervening generations don't light up. Following recent reports that stress during pregnancy can be passed on for four generations this is not surprising, but the changes observed were not always in the direction that might be expected.

In the American Journal of Human Biology Professor Jean Golding and colleagues at the University of Bristol report on a study of physical characteristics of children 7-17. They divided the children into four categories based on whether their mothers, and their maternal or paternal grandmothers, smoked during pregnancy.

Girls whose paternal grandmother smoked during pregnancy, after controlling for factors such as education and whether the father smoked, were taller than their peers. Both boys and girls with smoking paternal grandmothers had greater bone and lean mass, and the boys showed increased strength and fitness as they aged.

Given the overwhelmingly negative effects of smoking, particularly during pregnancy, the connection is unclear. The study was inspired by one conducted in Överkalix, Sweden, near the Arctic Circle, where extensive information about the experiences of children growing up in the late 19th and early 20th Centuries demonstrated that periods of good and bad food supply left a mark two generations later. 

However, the authors point out, “In the UK, since the Second World War, there have been no particular years of starvation or glut.” Consequently they chose smoking as the environmental factor most likely to leave a mark down the track.

The sample group included 8290 children tested at seven, with more than 5000 of these still participating in biennial checks at 17. 

Nevertheless, the fact that only a few of the characteristics measured showed a significant correlation, and that some of these were counter-intuitive, does raise questions about the reliability of the findings. The authors themselves say, “Explaining the sex differences is a challenge.” Although the paper speculates that these differences may result from a parental genetic tug-of-war, it does not discuss why smoking during pregnancy should produce traits generally thought of as beneficial in grandchildren.

A year ago a paper in The American Journal of Physiology produced a more negative take on the effects of smoking and took it one generation further, albeit in rats. The offspring of rats given nicotine during pregnancy had an “asthma-like phenotype” and this was passed on to the great-grandchildren of the original test subjects. That study concluded that nicotine was affecting lung cells in ways that produced asthma, and sex cells such that this characteristic was passed on down the line.