Smoking during pregnancy is twice as bad as previously thought, with a threefold risk of premature birth and with babies born to these mothers being four times more likely to be small-for-date, as well as being at more risk of serious complications. Caffeine, however, had no measured ill effects.
Additionally, reports The Guardian, the study suggested that tea and coffee do not harm babies, although it is recommended that pregnant women drink no more than 200mg of caffeine a day (two cups of instant coffee or tea).
Although drinking large amounts of caffeine and smoking during pregnancy have been tied to an increased risk of complications, premature birth and foetal growth restriction, the study by Cambridge academics found no evidence that above-average caffeine intake throughout pregnancy was linked to pre-term birth or smaller babies.
In contrast, the research calculated that the smokers were nearly three times more likely to give birth prematurely compared with non-smokers – more than double the previous estimate.
The findings come as a separate study by the University of Essex found that a pregnant woman or partner losing their job was associated with an increased risk of miscarriage or stillbirth.
Previous studies looking at the links between smoking, caffeine and adverse pregnancy outcomes have tended to rely on self-reported data to estimate exposure, often at a single time point in pregnancy, which is not always reliable.
This research, published in the International Journal of Epidemiology, examined metabolites from smoking (cotinine) and caffeine (paraxanthine) in the blood samples of more than 900 women who took part in the Pregnancy Outcome Prediction study between 2008 and 2012.
Researchers analysed levels of cotinine and paraxanthine found in the blood at 12, 20, 28 and 36 weeks of pregnancy, and classified exposure to smoking and caffeine consumption accordingly.
Prof Gordon Smith, head of the department of obstetrics and gynaecology at the University of Cambridge, said the findings underlined the importance of smoking cessation services.
“We’ve known for a long time that smoking during pregnancy is not good for the baby, but our study shows it’s potentially much worse than previously thought,” he said. “It puts the baby at risk of potentially serious complications from growing too slowly in the womb or from being born too soon.”
But Smith said the findings did not mean that pregnant women should start drinking lots of espressos. Studies have shown that very high intake of caffeine, in particular coffee, is linked to a higher risk of miscarriage and stillbirth, which were not included in this research, he pointed out.
“We only studied women who consistently had above average levels of caffeine metabolites. We did not study women who had extremely high caffeine intake.
“Hence, we would not interpret our findings as indicating that current recommendations around caffeine intake should be changed.”
Study 1 details
Objective measures of smoking and caffeine intake and the risk of adverse pregnancy outcomes
Roshan Selvaratnam, Ulla Sovio, Emma Cook, Francesca Gaccioli, Stephen Charnock-Jones, Gordon Smith.
Published in the International Journal of Epidemiology on 28 September 2023
Abstract
Background
In pregnancy, women are encouraged to cease smoking and limit caffeine intake. We employed objective definitions of smoking and caffeine exposure to assess their association with adverse outcomes.
Methods
We conducted a case cohort study within the Pregnancy Outcome Prediction study to analyse maternal serum metabolomics in samples from 12, 20, 28 and 36 weeks of gestational age. Objective smoking status was defined based on detectable cotinine levels at each time point and objective caffeine exposure was based on tertiles of paraxanthine levels at each time point. We used logistic and linear regression to examine the association between cotinine, paraxanthine and the risk of pre-eclampsia, spontaneous pre-term birth (sPTB), foetal growth restriction (FGR), gestational diabetes mellitus and birthweight.
Results
There were 914 and 915 women in the smoking and caffeine analyses, respectively. Compared with no exposure to smoking, consistent exposure to smoking was associated with an increased risk of sPTB [adjusted odds ratio (aOR) = 2.58, 95% CI: 1.14 to 5.85)] and FGR (aOR = 4.07, 95% CI: 2.14 to 7.74) and lower birthweight (β = –387 g, 95% CI: –622 g to –153 g). On univariate analysis, consistently high levels of paraxanthine were associated with an increased risk of FGR but that association attenuated when adjusting for maternal characteristics and objective—but not self-reported—smoking status.
Conclusions
Based on objective data, consistent exposure to smoking throughout pregnancy was strongly associated with sPTB and FGR. High levels of paraxanthine were not independently associated with any of the studied outcomes and were confounded by smoking.
Study 2 details
Job loss during pregnancy and the risk of miscarriage and stillbirth
Alessandro Di Nallo, Selin Köksal.
Published in Human Reproduction on 27 September 2023
Abstract
Question
Does the exposure to job loss during pregnancy increase the risk of miscarriage or stillbirth?
Summary answer
The experience of own or partner’s job loss during the pregnancy is associated with an increased risk of miscarriage and stillbirth.
What is known already
Prior research on the psycho-social aspect of pregnancy loss has investigated the contextual and the individual-level stressors. At the contextual level, natural disasters, air pollution, and economic downturns are associated with higher risk of pregnancy loss. At the individual level, intense working schedules and financial strain are linked with increased risk of pregnancy loss both at early and later stages of the gestation.
Study design, size and duration
This work draws on high-quality individual data of ‘Understanding Society’, a longitudinal survey that has interviewed a representative sample of households living in the UK annually since 2009. Approximately 40 000 households were recruited. The analyses use all the available survey waves (1–12, 2009–2022).
Participants/materials, settings, methods
The final sample consisted of 8142 pregnancy episodes that contain complete information on pregnancy outcome and date of conception. Ongoing pregnancies at the time of the interview were excluded from the final sample. The outcome variable indicated whether a pregnancy resulted in a live birth or a pregnancy loss whereas the exposure variable identified the women’s or their partner’s job loss because of redundancy or a dismissal. Logistic regression models were employed to estimate the relation between job loss during pregnancy and pregnancy loss. The models were adjusted for an array of socio-demographic and economic characteristics following a stepwise approach. Several sensitivity analyses complemented the main findings.
Results and role of chance
Baseline models controlling for women’s demographic background and prior experience of miscarriage estimated an increased risk of pregnancy loss when women were exposed to their own or their partner’s job loss during their pregnancy (odds ratio (OR) = 1.99, 95% CI: 1.32, 2.99). When the models were adjusted for all socio-economic and partnership-related covariates the association remained robust (OR = 1.81, 95% CI: 1.20, 2.73).
Limitations
First, the pregnancy outcome and the date of conception were self-reported and may be subjected to recall and social desirability bias. Second, although we adjusted for an array socio-demographic characteristics and self-reported health, other contextual factors might be correlated with both job loss and pregnancy loss. Third, owing to the limited sample size, we could not assess if the main finding holds across different socio-economic strata.
Wider implications
By showing that exposure to a job loss during pregnancy increases the risk of miscarriage and stillbirth, we underline the relevance of pregnancy loss as a preventable public health matter. This result also calls for policy design that enhances labour market protection and social security buffers for pregnant women and their partners.
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