There can be little doubt about the role of smoking in lung cancer and cardiovascular disease. But how significant is it in women who are pregnant?

A retrospective study has found a link between smoking and stillbirth by carefully looking at stillbirth rates in women for whom data were available for two consecutive pregnancies.

Sweden has an excellent nationwide birth registry, which includes data on a number of variables measured and collected before 15 weeks of pregnancy. The researchers wished to determine whether stillbirth rates changed if the smoking habits of the mothers changed between two consecutive pregnancies. The data analysis used a multiple regression process to control for such relevant variables as maternal age, education, interpregnancy interval and whether the mother was Nordic.

A total of 526,691 women were included in the study.

The overall stillbirth rate in the second pregnancy of women who did not smoke in either pregnancy was 2.4 per 1,000 pregnancies. The rate rose to 2.8 per 1,000 and 4.1 per 1,000 respectively in women who smoked moderately (fewer than 10 cigarettes a day) or heavily (more than 10 cigarettes a day) in both pregnancies.

In moderate smokers who became heavy smokers in the second pregnancy the rate of stillbirth was 4.0 per 1,000 pregnancies. However, in heavy smokers who became moderate or non-smokers in the second pregnancy the stillbirth rates dropped to 3.6 and 2.1 per 1,000 pregnancies respectively.

Curiously the rate did not change in moderate smokers who quit by the second pregnancy. Various explanations for this unexpected finding were proposed, not least that smoking habits may have changed in the second pregnancy after the time of data collection.

It is known that smokers have lighter babies and are more likely to suffer placental abruption. The research team concluded that smoking is an additional independent and significant cause of stillbirth.

The message is that cessation from heavy smoking reduces the risk of stillbirth in subsequent pregnancies, but also that smoking reduction reduces the risk too.

Hogberg L, Cnattingius S. The influence of maternal smoking habits on the risk of subsequent stillbirth: is there a causal relation? BJOG 2007;113:699-704