Impact of a stepwise introduction of smoke-free legislation on the rate of preterm births: analysis of routinely collected birth data

BMJ 2013; 346 doi: http://dx.doi.org/10.1136/bmj.f441 (Published 14 February 2013)
Cite this as: BMJ 2013;346:f441

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With great interest we read the report by Cox et al. on the impact of a step-wise introduction of smoke-free legislation on the rate of preterm births (1). The authors found a significant reduction in the risk of spontaneous and overall preterm delivery in Flanders Belgium since the ban on smoking in restaurants on January 1st 2007 and in bars serving food on January 1st 2010. The authors welcome further proof of their observations from countries where smoking bans have been relaxed, as in The Netherlands.

In The Netherlands, the smoking ban legislation was implemented on January 1st 2004 for public places and workplaces and on July 1st 2008 for restaurants, bars and cafés. To compare the findings of Cox et al. with the situation in The Netherlands, being the immediate neighbour of Belgium, we used data from our national Perinatal Registry between 2000 and 2009. We used the same inclusion criteria being singleton, live born infants delivered at 24–44 weeks of gestation (n= 1.726.180). We found an overall reduction in the rate of spontaneous (from 3.9 to 3.0%) and overall (from 6.0% to 5.7%) preterm birth since 2000. No additional effect was seen after the introduction of the smoke free legislation in 2004 and/or 2008 (see Figure).

A possible explanation could be that confounders, other than on an individual or population level, were responsible for the observed results of Cox et al. The authors themselves touch on changes in therapeutic strategies, such as the prescription of atosiban and on cervical cerclage treatment. In a rapid response on February 23rd 2013, Page suggests that the introduction of cervical length measurement by vaginal ultrasound and the consequent implementation of intravaginal progesterone to prevent preterm delivery in the same time frame as the smoking ban legislations might provide a better explanation for the pattern of risk reduction of preterm delivery (2). Although we agree that smoking, both active and passive, has a negative effect on health and pregnancy, our data does not confirm that smoke-free legislation had an impact on preterm birth in The Netherlands.

Myrthe Peelen, MD
Petra Hajenius, MD PhD
Ben Willem Mol, MD PhD
Anita Ravelli, MSc PhD

References
1. Cox, B., Martens, E., Nemery, B., Vangronsveld, J., Nawrot, T. Impact of a stepwise introduction of smoke-free legislation on the rate of preterm births: analysis of routinely collected birth data. BMJ 2013;346:f441
2. Page, G.H. Re: Impact of a stepwise introduction of smoke-free legislation on the rate of preterm births: analysis of routinely collected birth data. BMJ [Internet]. 2013 Feb [cited 2013 March 20]. Available from: http://www.bmj.com/content/346/bmj.f441?tab=responses

Figure: Rate of spontaneous and overall preterm birth in The Netherlands 2000-2009

Competing interests: None declared

Myrthe JCS Peelen, PhD student Obstetrics & Gynaecology

Petra Hajenius, Ben Willem Mol, Anita Ravelli

Academic Medical Centre, Meibergdreef 9, 1105 AZ, Amsterdam

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Dear Editor,
although a good statistical exercise,this paper fails to prove a causality betwen smoke-free legislation and a reduction of mild and moderate (but non extreme) preterm births!
As the authors mentioned,there is a lack of individual data,the fluctuations of reduction of preterm births over a ten years period (-0.7%) are small and a lot of confounding variables were overlooked.
Possibly, the most important of these is the introduction of cervical length measurement by vaginal ultrasound and the consequent implementation of intravaginal progesterone administration in Flanders.
This is a much more important therapeutic strategy to take into account than Atosiban and cerclage which are mainly therapeutic interventions considered after preterm labour has already started.
Although smoking banning is good for public health,too much merit is attributed to the impact of this intervention to the reduction in preterm births.In this era of evidence based medicine, more analysis is needed before it might be confirmed that smoke-free legislation has an impact on preterm birth. However, the media will do this to the front as if smoke-banning has an important impact on the decreasing trend of preterm births.

Competing interests: None declared

Geert H Page, Obstetrician

Jan Yperman Hospital, Briekestraat 12 Ypres, Belgium

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I am quite surprised by the fact that smoking is related to Fetal Growth Restriction (1,2)apart from preterm births. Authors could have observed and discussed this as well. Studying low birth weights (a composite indicator of Fetal Growth Restriction and preterm delivery) could have been studied too. These variables would have added value to the adverse repercussions of effect of smoking in pregnancy outcomes.

References:

1: Walfisch A, Nikolovski S, Talevska B, Hallak M. Fetal growth restriction and
maternal smoking in the Macedonian Roma population: a causality dilemma. Arch
Gynecol Obstet. 2013 Jan 30. [Epub ahead of print]

2: Koch S, Vilser C, Groß W, Schleußner E. [Smoking during pregnancy: risk for
intrauterine growth retardation and persisting microsomia]. Z Geburtshilfe
Neonatol. 2012 Apr;216(2):77-81. doi: 10.1055/s-0032-1308958. Epub 2012 Apr 19.
German.

Competing interests: None declared

Neeru Gupta, Scientist E

KK Jani, Jugal Kishore

Indian Council of Medical Research, Ansari Nagar, New Delhi-110029

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Correlations in time trends are not easily interpreted. “Victory has many fathers, defeat remains an orphan. (Clausewitz)”: favorable time trends find rapidly explanations, desirable to policy makers, unfavorable trends are gladly ignored.

A “run in period” of four years (2002-2006) before the change is short. There is no apparent reason why the authors choose a start date so close to the trend change in 2006. A brief look at the Flemish birth rates show that the year of 2002 coincides with a rather sudden change from a long standing decrease to a relatively pronounced increase in birth rates. The denominator is increasing far more than the numerator is decreasing. This is partly caused by an increasing fertility and increasing birth order in more recent periods in Flanders.

The authors refer to the paper of Keirse et al (1), but fail to inform the reader on the time trends before 2002, cited in this paper: these increased from 5.4% to 7.2%, from 1991-2002, a more dramatic increase than the decrease from 7.4% to 6.7% from 2006 till 2012. This "defeat", the increase of preterm births in Flanders, remains largely unexplained.

As a non-smoker, I support a smoking ban in all public places, but I would suggest that the results of this paper are an isolated small fluctuation in poorly explained trends of preterm births in Flanders.

1. Keirse MJNC, Hanssens M, Devlieger H. Trends in preterm births in Flanders, Belgium, from 1991 to 2002. Paediatr Perinat Epidemiol2009;23:522-32.

Competing interests: None declared

Bonneux Luc, MD elderly care, epidemiologist

NA, Termikkelaan 22, 2530 Boechout

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