The interrelationship of maternal smoking and increased perinatal mortality with other risk factors. Further analysis of the Ontario Perinatal Mortality Study, 1960-1961.

Increased perinatal mortality among smokers' babies has been observed in many but not in all studies, with a statistically significant difference in some. This paper explores the hypothesis that maternal smoking may interact with other risk factors, so that a dose-related increase in perinatal mortality may be enhance or masked depending upon the presence or absence of these factors. Data are from the Ontario Perinatal Mortality Study of all single b irths in 10 teaching hospitals in Ontario in 1960-1961 a total of 51,490 births, including 701 fetal deaths and 655 early neonatal deaths. Perinatal mortality increased significantly with smoking, and was also affected by such factors as maternal age, parity, hospital status, previous pregnancy history, hemoglobin level, and others. Smoking frequencies also varied by many of these characteristics. Perinatal mortality was therefore analyzed by the amount smoked during pregnancy within subgroups of these antecedent risk factors. When smoking and other risk factors were cross-tabulated among 52 data subgroups, only the light smokers (less than 1 pack per day) under age 20 had lower perinatal mortality rates than their nonsmoking counterparts. In almost all subgroups the mortality increase with smoking was dose-related, but not in a simple, linear way. The increased risk of perinatal mortality associated with light smoking among young, low-parity, non-anemic mothers was less than 10 percent. At the other extreme, mothers of high parity, public hospital status, with previous low birthweight births, or with hemoglobin less than 11 gm had increased perinatal mortality risks of 70-100 percent when they were heavy smokers. The failure of some studies to find a significant increase in perinatal mortality with maternal smoking may be due to selection of study populations from the end of the spectrum where light smoking is associated with only a slight increase in perinatal risk. Other studies may select higher risk populations, where the influence of smoking on mortality is stronger. Depending on the magnitude of the difference, the amount smoked, and the size of the study, results might or might not be statistically significant.