Smoking During Pregnancy: The Risk of Premature Birth is More Than Twice as High

Research shows that women who smoke during pregnancy are 2.6 times more likely to have a premature baby than non-smokers—more than twice as likely as previously thought. The study, published in the International Journal of Epidemiology, also found that smoking increases the likelihood of the baby being too small for its gestational age by four times, putting it at risk of potentially serious complications such as respiratory problems and infections. However, the team found no evidence that caffeine consumption is associated with any negative effects.

Smoking During Pregnancy and the Dangers

Women are strongly advised to stop smoking during pregnancy and limit their caffeine intake, as this reduces the risk of complications for the baby.

For example, smoking during pregnancy is associated with an increased risk of fetal growth retardation, premature birth, and low birth weight, although it is also associated with a reduced risk of preeclampsia (high blood pressure during pregnancy).

High caffeine consumption is also associated with lower birth weight and possibly fetal growth retardation. Caffeine is more difficult to avoid than cigarette smoke, as it is found in coffee, tea, chocolate, energy drinks, soft drinks, and certain medications. Studies examining the association between smoking, caffeine, and adverse pregnancy outcomes typically rely on self-reported data to estimate exposure, which is not always reliable. A more objective measurement method is to examine the concentration of metabolites in the blood—chemical byproducts that are produced when substances such as tobacco and caffeine are processed in the body.

Probability of Spontaneous Preterm Birth 2.6 Times Higher

Researchers at the University of Cambridge and Rosie Hospital, part of Cambridge University Hospitals NHS Foundation Trust, recruited more than 4,200 women who attended the hospital between 2008 and 2012 as part of the Pregnancy Outcome Prediction (POP) study. The team analyzed blood samples taken from a subset of these women four times during their pregnancies. To assess exposure to cigarette smoke, the team examined the levels of the metabolite cotinine, which can be detected in blood, urine, and saliva. Only two out of three women with detectable cotinine levels in each blood sample reported being smokers, showing that this measurement is a more objective method of assessing smoking behavior.

A total of 914 women were included in the smoking analysis. Of these, 78.6% were classified as not exposed to smoking during pregnancy, 11.7% as partially exposed, and 9.7% as constantly exposed. Compared to women who were not exposed to nicotine during pregnancy, women with constant exposure were 2.6 times more likely to have a spontaneous preterm birth—more than twice the previous estimate of 1.27 from a meta-analysis of studies—and 4.1 times more likely to have fetal growth restriction.

Babies born to smokers weighed on average 387 g less than babies born to non-smokers – more than 10% less than the average weight of a newborn. This increases the risk of the baby having a low birth weight (2.5 kg or less), which in turn is associated with an increased risk of developmental disorders and poorer health later in life. However, unlike previous studies, the team found no evidence that smoking reduces the risk of preeclampsia.

Little Evidence of a Link Between Caffeine Intake and Negative Effects

Professor Gordon Smith, head of the Department of Obstetrics and Gynecology at the University of Cambridge, said: “We have long known that smoking during pregnancy is not good for the baby, but our study shows that it may be much worse than previously thought. It exposes the baby to the risk of potentially serious complications because it grows too slowly in the womb or is born too early.

“We hope that this knowledge will encourage pregnant mothers and women planning to become pregnant to take advantage of smoking cessation services. Pregnancy is a crucial time for women to quit smoking, and if they can remain tobacco-free after giving birth, it will have lifelong benefits for them and their child.”

To assess caffeine intake, the researchers examined the metabolite paraxanthine, which accounts for 80% of caffeine metabolism and is both less sensitive to recent intake and more stable throughout the day. 915 women were included in the caffeine analysis. Of these women, 12.8% had low paraxanthine levels throughout pregnancy (indicating low caffeine intake), 74.0% had moderate levels, and 13.2% had high levels. There was little evidence of an association between caffeine intake and adverse effects.

Women who have difficulty giving up nicotine should definitely talk to their doctor. There are special counseling services for pregnant women that are gentle and supportive. Free smoke-free hotlines or apps can also provide motivation.