Tobacco smoke contains thousands of compounds, many of them toxic and capable of causing injury throughout the body. Because of this high toxicity of tobacco smoke, many diseases have long been causally linked to tobacco smoking – both to active smoking and to passive exposure of non-smokers who inhale the mixture of exhaled smoke and smoke given off by the smouldering cigarette, generally referred to as secondhand smoke or SHS.

The journal Tobacco Control recently published findings on smoking and pregnancy outcomes from a very large US study, the Women’s Health Initiative. The authors assessed whether active smoking by the mother while pregnant and exposure of non-smoking mothers to SHS led to an increased risk of spontaneous abortion, stillbirth and tubal ectopic pregnancy (that is, implantation of the fertilised egg into the fallopian tube, rather than the uterus). Overall, the study found that both active smoking and SHS exposure increased all of these risks for pregnancy.

There have been many studies in the past, some as long as half a century ago, on smoking and pregnancy. The results are clear and definitive: smoking by the mother while pregnant harms the child in a number of ways including reducing birth weight, by several hundred grams on average, and increasing the miscarriage rate through a number of mechanisms.

The recent 50th anniversary report of the US Surgeon General on smoking added new conclusions that smoking by the mother during pregnancy increases risk for ectopic pregnancy and for orofacial clefts and may have long-range implications for the child’s neuropsychological development. With regard to SHS exposure during pregnancy, an association with a small reduction of birth weight has also been found and Sudden Infant Death Syndrome (SIDS), or cot-death, is causally associated with this exposure.

The new report on smoking and pregnancy outcome uses data collected at entry into the Women’s Health Initiative Observational Study from 80,672 women. They were queried about their own smoking and exposure to SHS across their lives; they were also asked about the outcomes of their pregnancies.

The authors analysed these data, collected in the 1990s, to assess whether the three adverse outcomes of pregnancy were associated with smoking, giving consideration to other factors. For women who had smoked during their reproductive years, there were significant associations with spontaneous abortion, stillbirth, and ectopic pregnancy with increases of 16%, 44% and 43%, respectively.

For pregnancy outcomes in women who never smoked but were exposed to SHS, the results were mixed, but the most exposed women had small but significantly increased risks for the three outcomes.

The authors acknowledge limitations of their research, which largely reflect the use of data collected in the past by self-report and at one point in time. That is, the researchers did not observe the women over time, monitoring for adverse pregnancy outcomes, but instead used recollection of past events and of smoking and SHS exposure.

Bias from flawed recall is of concern but the findings are generally consistent with prior reports that have included follow-up and better documentation of smoking and pregnancy outcome. The authors opportunistically used available data from a very large study and the study size allowed them to quantify risks that are known to be subtle but important from a societal perspective.

So, in spite of inherent limitations of the data, the new study confirms well-documented adverse consequences of smoking by the mother for the foetus and indicates that even the lower doses of smoke received by the foetus from SHS exposure of the mother may be harmful.

There has long been a sufficient and highly compelling scientific rationale for protecting the foetus from tobacco smoke exposure. In many countries, women stop smoking while pregnant but not all do so. In the United States, for example, there are about 500,000 births a year among women who smoked while pregnant. To answer the question: does smoking harm the foetus?, the answer is clearly “yes”, and prenatal care must involve aggressive attention to active and passive smoking in pregnant women.