Pregnancy and child

Premature births 'fell 10% after smoking ban'

Scotland’s premature birth rate has fallen by 10% since the public smoking ban came into force in 2006, BBC News reported today.

The news is based on the results of a large Scottish study that looked at trends in numbers of premature births and small babies born between 1996 and 2009, and how these related to the introduction of the smoking ban in March 2006. The researchers found that there was a decline in the number of premature births in the three months before the introduction, but since then there has been a slight fluctuation and numbers have begun to rise again overall. Conversely, the number of babies born small for the length of time they were in the womb declined around 2006, and has generally continued to fall.

Smoking is a known risk factor for premature birth and babies born small for the length of time they were in the womb (gestational age), and this research provides valuable clues to the potential impact of the smoking ban. However, the study only found trends, which means it cannot prove the legislation caused the drop in rates seen. It is possible that other factors may be responsible, such as general improvements in antenatal care.

Both smoking in pregnancy and passive smoking are associated with a higher risk of premature birth, babies born small and other pregnancy complications.

Where did the story come from?

The study was carried out by researchers from University of Glasgow and Western General Hospital, Edinburgh. It was funded by Scotland’s Chief Scientist Office. The study was published in the peer-reviewed medical journal PLoS Medicine.

It was reported accurately by the BBC, which pointed out that other factors might have influenced the results.

What kind of research was this?

This time-trend study looked at the numbers of babies born prematurely or small for gestational age in Scotland before and after the introduction of the smoking ban in March 2006. It looked at data on babies born to nearly 717,000 pregnant women between 1996 and 2009.

The researchers examined both trends in data and the possible impact of the legislation in Scotland. However, while this type of study can identify trends, it cannot confirm the various factors that caused the trends. It examined how trends related to both “active” and “passive” smoking, also known as first-hand and second-hand smoking respectively.

Both active and passive smoking during pregnancy are known to increase the risk of various complications. The researchers say the legislation – the Smoking, Health and Social Care (Scotland) Bill – has been very successful in reducing exposure to environmental tobacco smoke (ETS) in public places. It has also been associated with greater voluntary restrictions on smoking in the home. They say there was an increase in attempts to quit among current smokers three months before the legislation was introduced, and a reduction in the amount smoked by those who continued to smoke.

What did the research involve?

The researchers gathered data from a national administrative database on pregnancy, which collects information on all women discharged from Scottish maternity hospitals and records many factors, including pregnancy complications and smoking status. Data on smoking status were based on women’s self-reported smoking habits, which were reported as “current”, “never” and “former” smokers. The researchers obtained data on all singleton, live-born babies delivered at 24 to 44 weeks of pregnancy between January 1996 and December 2009. They used postcodes as an indicator of the women’s socioeconomic status.

From this data, they collected information on the rates of two complications of pregnancy: babies born small for gestational age and premature delivery. Babies were classed as small for gestational age if their birth weight was within the lowest 10% of babies of the same sex born at the same point in pregnancy. Premature delivery was defined as delivery before 37 weeks of pregnancy, and was categorised as:

  • mild - between 34 and 37 weeks
  • moderate - between 32 and 34 weeks
  • extreme - earlier than 32 weeks

The researchers also looked at other outcomes, such as spontaneous premature delivery (as opposed to overall premature deliveries which would include those that were planned, for example premature induced labour or caesarean due to complications with the mother or baby).

Researchers looked at the trends in these outcomes before and after the introduction of smoking legislation. They were particularly interested in two time points: the date when the legislation was implemented (March 26 2006), and three months before (January 1 2006). The latter date, they explain, allows for the possibility of people making changes to smoking behaviour in anticipation of the legislation and was chosen because it coincided with a New Year peak in attempts to quit smoking found in a previous study.

In their analysis, the researchers adjusted their results to account for other factors that may affect these pregnancy outcomes, including maternal age, sex of the infant and socioeconomic factors.

What were the basic results?

The researchers included 716,941 women who fulfilled all their criteria and for whom they had information on smoking status. They found that the number of current smokers fell from 25.4% before legislation to 18.8% after legislation. From looking at the trends in numbers of babies born prematurely or small for gestational age, they noticed that, of the two dates, January 1 2006 (three months before the smoking ban) seemed to have a greater influence on the fall.

The graph depicting the trend in premature births between 1996 and 2009 shows a general fluctuation in rates. Around January 2006, there is an apparent decline in rates, but over the following three years, there has been continued fluctuation and numbers have begun to rise again. For the number of babies born small for gestational age, there was a similar decline around 2006. However, the trend, though still fluctuating, seems to have continued downwards since then, rather than rising as it did with premature births.

The researchers report that, after January 1 2006:

  • The number of babies born small for gestational age fell by 4.52% (95% confidence interval [CI] -8.28 to -0.60).
  • The number of babies born prematurely fell by 11.72% (95% CI -15.87 to -7.35).
  • The number of babies born after spontaneous premature labour fell by 11.35% (95% CI -17.20 to -5.09).

These significant reductions were found among all women, including those who still smoked and those who had never smoked.

How did the researchers interpret the results?

The researchers say that three months before the introduction of the new legislation, the numbers of premature deliveries and babies born small for gestational age fell significantly, although they point out that rates of premature births have since begun to rise again. They say this is consistent with a previous study which showed that smokers anticipated legislation, resulting in a significant peak in prescriptions for nicotine replacement therapy in January 2006.


Overall, this analysis of the relationship between pregnancies and the smoking ban in Scotland provides a valuable insight into the possible results of anti-smoking legislation. In particular, the decline in rates of premature births and babies born small for gestational age around January 2006 is interesting. As smoking is a known risk factor for these outcomes, the trend could be the result of higher quit rates, both among pregnant women or the public in general, in anticipation of the new law.

However, the trend analysis performed in this study cannot prove there is a definite relationship between the two, but only that there are associations. It is possible that other factors are involved, such as general improvements in antenatal care and management of pregnant women who are at risk of these complications. Furthermore, there has been continuing fluctuation in numbers of babies born prematurely or small for gestational age since the smoking ban in 2006. The subsequent general increase in premature births makes it even harder to draw any conclusions about the reasons behind this trend.

A further limitation of the study was that women’s smoking status was based on them reporting whether or not they smoked. As the authors point out, there is evidence that pregnant women underestimate how much they smoke and it is possible they felt under pressure to conceal their smoking following the new law. However, this would not affect the overall results of the study, which related to all deliveries irrespective of smoking status.

The study cannot prove that smoking legislation – or anticipation of it – reduced the risk of pregnancy complications. Nevertheless, smoke-free legislation is now recognised as having health benefits and it is plausible that improved pregnancy outcomes are one of them.

NHS Attribution