Premature birth risk from smoking while pregnant doubles

The risk of giving birth prematurely due to smoking during pregnancy is more than double that than previously believed, researchers report today.

The study, led by researchers at Cambridge University, UK, said not only are smokers during pregnancy 2.6 times more likely to give birth prematurely compared to non-smokers, but that the baby was four times more likely to be small for its gestational age.

Writing in the International Journal of Epidemiology, they say there was no evidence that caffeine intake was linked to adverse outcomes.

Women are recommended to stop smoking and limit their caffeine intake during pregnancy because of the risk of complications to the baby, such as foetal growth restriction, premature birth and low birthweight.

Although there have been studies that have examined the link between smoking, caffeine and adverse pregnancy outcomes, they rely mainly on self-reported data. A more objective measure is to look at levels of metabolites in the blood, the researchers say.

More than 4,200 women who attended the Rosie Hospital, part of Cambridge University Hospitals NHS Foundation Trust hospital, between 2008 and 2012 took part in the Pregnancy Outcome Prediction (POP) study.

The researchers, from the university and hospital, analysed blood samples taken from a subset of these women four times during their pregnancies.

Levels of the metabolite cotinine, which can be detected in blood, urine, and saliva, were analysed to assess exposure to cigarette smoke, and two out of three women with detectable levels in every blood sample were self-reported smokers. This, say the researchers demonstrates this measure is a more objective way of assessing smoking behaviour.

In total, 914 women were included in the smoking analysis, of whom 78.6% had no exposure to smoking while pregnant, 11.7% had some exposure and 9.7% had consistent exposure.

Compared to women who were not exposed to smoking while pregnant, those with consistent exposure were 2.6 times more likely to experience spontaneous preterm birth – more than double the previous estimate of 1.27 from a meta-analysis of studies – and 4.1 times as likely to experience foetal growth restriction.

Babies born to smokers were on average 387g lighter than babies born to non-smokers, approximately 10% smaller than the weight of an average newborn.

Unlike in previous studies, however, the team found no evidence that smoking reduced the risk of pre-eclampsia.

Professor Gordon Smith, head of the department of obstetrics and gynaecology at the University of Cambridge, said: “We’ve known for a long time that smoking during pregnancy is not good for the baby, but our study shows that it’s potentially much worse than previously thought. It puts the baby at risk of potentially serious complications from growing too slowly in the womb or from being born too soon.

“We hope this knowledge will help encourage pregnant mums and women planning pregnancy to access smoking-cessation services. Pregnancy is a key time when women quit and if they can remain tobacco free after the birth there are lifelong benefits for them and their child.”

To assess caffeine intake, researchers analysed the metabolite paraxanthine, which accounts for 80% of caffeine metabolism and is both less sensitive to recent intake and more stable throughout the day.

For this analysis, 915 women were included and 12.8% had low levels of paraxanthine throughout pregnancy, 74.0% had moderate levels and 13.2% had high levels. There was little evidence of an association between caffeine intake and any of the adverse outcomes.

Meanwhile, new analysis from Cancer Research UK published today shows that if trends continue, the poorest communities are not projected to be smokefree by 2050 – 27 years after the richest.

The charity is urging the UK government to make smokefree, which is defined as 5% or less of the adult population smoking, priority for all – and not leave the most deprived behind.

Cancer Research UK’s chief executive, Michelle Mitchell said: “It’s appalling to see that the most deprived communities in England are not projected to be smokefree until after 2050. There are nearly twice as many cancer cases caused by smoking in the poorest areas compared to the wealthiest in England. It’s been reported that the Prime Minister is considering raising the age of sale of tobacco – this has broad support and is something we would welcome. But we still need tangible action to help people who smoke quit.

“The Autumn Statement could be the perfect opportunity for Jeremy Hunt, with cross-party support, to make the tobacco industry pay for the damage it causes. To give our loved ones the proper support they need. All political parties should agree on the critical steps to bring an end to smoking for everyone.”

The smokefree target for 5% smoking prevalence is 2030 for England. Scotland and Wales have their own targets of 2034 and 2030 respectively, while Northern Ireland currently has no smokefree target.

Selvaratnam RJ, Sovio U, Cook E et al. Objective measures of smoking and caffeine intake and the risk of adverse pregnancy outcome. International Journal of Epidemiology 28 September 2023; doi: 10.1093/ije/dyad123

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