THURSDAY, Sept. 28, 2023 (HealthDay News) -- Smoking during pregnancy is a significant risk factor for premature births, but drinking coffee is not, new research suggests.

Women who smoked during pregnancy were 2.6 times more likely to give birth prematurely compared to nonsmokers, a risk that was double that of previous estimates, the University of Cambridge scientists found.

“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,” said Gordon Smith, head of the department of obstetrics and gynecology at Cambridge, in the United Kingdom.

The study also showed that with prenatal smoking, a baby was four times more likely to be small for its gestational age. This brings the risk of potentially serious complications, including breathing difficulties and infections.

On the other hand, high caffeine intake from coffee or other drinks did not have the same effect, even though it's been shown previously to be associated with lower birth weights and possibly fetal growth restriction.

In this study, scientists measured levels of chemical byproducts created when substances such as tobacco and caffeine are processed in the body.

The investigators recruited more than 4,200 women who attended the Rosie Hospital, part of Cambridge University Hospitals NHS Foundation Trust, between 2008 and 2012 as part of the Pregnancy Outcome Prediction (POP) study.

The scientists analyzed blood samples taken from a subset of these women four times during their pregnancies. They looked at levels of the metabolite cotinine to assess smoke exposure. This can be detected in blood, urine and saliva. Only two out of three women with detectable levels of cotinine in every blood sample were self-reported smokers.

Among the 914 women included in the smoking analysis, nearly 79% had no exposure to smoking while pregnant, about 12% had some exposure and 10% 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, and four times more likely to experience fetal growth restriction, the researchers reported.

The authors also found that babies born to smokers were an average of 387 grams (about 14 ounces) lighter than babies born to nonsmokers, which is about 10% smaller than the weight of an average newborn.

The team did not find evidence that smoking reduced the risk of preeclampsia, which had been seen in previous research.

“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,” Smith said in a university news release.

In the caffeine assessment, researchers looked for the metabolite paraxanthine, which accounts for 80% of caffeine metabolism.

Among the 915 women included in the caffeine analysis, 12.8% had low levels of paraxanthine throughout pregnancy, 74% 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.

The findings were published Sept. 27 in the International Journal of Epidemiology. The work was supported by the National Institute for Health and Care Research Cambridge Biomedical Research Centre and the Medical Research Council.

More information

The U.S. Centers for Disease Control and Prevention has more on preterm birth.

SOURCE: University of Cambridge, news release, Sept. 27, 2023