Thursday March 13 2008
“Women who give up smoking in pregnancy have more relaxed children”, The Independent reports today. It and other newspapers describe a recently published study of 18,000 babies that asked mothers about their smoking habits. It found that heavy smokers had the most difficult children, with the lowest scores for positive mood. The newspapers talk about previous animal research that has shown the harmful effects of nicotine as a behavioural toxin. However, they also raise the researchers’ suggestions that giving up smoking in pregnancy is associated with an urge to protect the baby and that this can lead to easy-going infants.
This research has raised the possibility that infants of mothers who quit smoking in pregnancy are more easy-going than either those who never smoked or those who are heavy smokers. However, these findings should be treated with caution: the difference between groups was very small and will need further investigation before clear advice can be given. It is already clear that quitting smoking is good for the baby, mother and those around them. Smoking during pregnancy is known to increase the risk of having a small baby, premature delivery or miscarriage.
Where did the story come from?
Dr Kate Pickett and colleagues from the Department of Health Sciences at the University of York and Dr Lauren Wakschlag from the Department of Psychiatry at the University of Illinois in Chicago carried out the research. The study was supported by the Medical Research Council. It was published in the Journal of Epidemiology and Community Health, a peer-reviewed medical journal.
What kind of scientific study was this?
This was a cross-sectional analysis of the data collected from the Millennium Cohort Study, a large prospective study of over 18,000 infants born between 2000 and 2002. This particular study used data that was collected from the infants when they were nine months old.
The parents gave detailed information on the pregnancy and birth of their baby, their health, and their social and economic circumstances. About 72% of those enrolled in the study responded to the questions at nine months. The researchers excluded those families with multiple births, such as twins, and those in which the person answering the questions was not the natural mother, e.g. the infant was adopted.
The researchers classified the mothers by their smoking habits as those who: never smoked during pregnancy, quit smoking during pregnancy, had been continuous light smokers during pregnancy (fewer than 10 cigarettes per day) or had been continuous heavy smokers during pregnancy (10 or more cigarettes per day).
Infant temperament was assessed using a questionnaire completed by the parents called the Carey Infant Temperament Scale. The Carey scale is an accepted, reliable and valid measure that scores parental answers to various questions on a scale of one to five. Three dimensions of temperament were assessed in this way: positive mood or cheerfulness, how receptive the infant was to novelty, and regularity in bodily functions (eating, nappy changes etc.). Higher scores indicated easier temperaments and the maximum score for these three indicators was 70.
The researchers then used statistical models to see if they could identify a link between smoking status and the component parts to the score, and any features in the background characteristics of mother or baby (such as ethnic group or birth weight) that could also influence the results.
What were the results of the study?
The researchers found that “pregnancy quitters had infants with the highest scores of easy temperament [57.3 on the Carey scale on average] and heavy smokers had infants with the lowest scores [55.7 on the Carey scale on average]”. The babies of women who never smoked had a score in between these, with an average of 56.7 on this scale.
When the researchers looked at the statistical models of the three component parts to the scale, heavy smoking was associated with an increased risk of negative mood. Pregnancy quitting exerted a protective effect, with the infants having a decreased risk of being distressed at novelty and irregularity.
What interpretations did the researchers draw from these results?
The researchers acknowledge that the associations between smoking during pregnancy and the behaviour of the offspring are complex and that temperament of infants may be determined by a host of other factors.
They suggest that their findings show that “both exposure and maternal characteristics associated with pregnancy smoking status contribute to offspring behavioural patterns”.
What does the NHS Knowledge Service make of this study?
This study collected data on a large number of infants and their mothers who were representative of the UK population. There are some aspects to the study that limit its interpretation:
- The absolute differences in scores between all groups were small. The overall difference across all groups was statistically significant, but there were fewer than three points between groups. This may not be important when taking into account the maximum score of 70 and the types of question asked.
- The mothers personally scored their infant’s temperament and an impartial observer did not validate this subjective assessment. There may be differences in the way parents reported their child’s temperament or answered questions relating to temperament that were also linked to the rates of maternal smoking. Factors such as the parents’ level of education, rather than smoking behaviour, may have influenced the results.
- The assessment of maternal smoking was brief, self-reported and retrospective. It did not capture the fluctuations in smoking that are common across pregnancy.
- Infant exposure to smoke from other people in the household was not assessed. This could have affected the results.
There may be important psychological differences between women who persist in smoking during pregnancy and those who quit. A better understanding of these may aid the design of pregnancy smoking-cessation programmes. Previous research has shown there are several increased risks associated with smoking in pregnancy such as having a small baby, premature delivery or miscarriage. Any of these is a good reason to stop smoking before pregnancy.