Smoking is deleterious and has negative effects on children born to mothers who smoke [12
]. We confirm a two-fold increased risk of SIDS in infants born to smoking mothers. In addition, as one cause of SIDS (prone sleeping) was reduced through the Back to Sleep
public health campaign in the United States, we noted an increasing attributed risk of SIDS with smoking. As a major etiology of SIDS, prone sleeping position, decreased over the study period due to the national educational campaign, the remaining SIDS deaths may now be due to the more isolated effects of tobacco exposure. Due to a decreased overall rate of SIDS, almost 80% of SIDS among smoke-exposed infants now bear a relationship to cigarette smoking.
Taylor and Sanderson, using data prior to the Back to Sleep
campaign in the United States, suggest that up to 30% of SIDS deaths may be attributed to maternal smoking [17
]. We note an increasing percent attributed risk in our cohort of 489,000 infants after the roll out of the campaign. Using the attributed risk to calculate an absolute mortality among the exposed cohort, 101 of 172 infant SIDS cases over the 10-year study period are linked to maternal smoking, which is greater than half of the infant deaths due to SIDS in the smoke-exposed cohort.
Our study cannot establish a causal role for maternal smoking in SIDS. Others have argued for a causal relationship based on prospective data, demonstrable dose-response relationships, and analysis using multiple potential confounders [18
]. The causal path appears more likely on the basis of multiple studies that support such a link, consistent findings of a dose-response relationship in the literature, and a potential biological basis for the association between the exposure and the outcome [19
]. In this study, removing one causative factor for SIDS, prone sleeping, may have increased the relative effect of another factor, maternal smoking, as an agent associated with SIDS.
The published literature reports a 20% to 30% smoking rate among pregnant women [20
]. Approximately 20% of pregnant smokers will deny smoking, but when tested with urine cotinine will be positive [22
]. A recent report suggests that 21% of women in Colorado are smokers [23
]. In our cohort, 15% of women reported some smoking during pregnancy, which may indicate a degree of under-reporting consistent with the published literature. While about one-fourth of women quit smoking during pregnancy, the recidivism rate is high and women are most likely to continue smoking into the postnatal period [24
]. Furthermore, as most individuals who smoke begin before the age of 18 years [27
], the results reported here provide yet another clarion call to eliminate smoking initiation and remove this preventable risk factor for infant death and SIDS. Expectant mothers, women desiring pregnancy, and health care providers who care for them need to be reminded about this strong association of infant death with the preventable risk factor of maternal smoking. For example, physicians can counsel women desiring or planning pregnancy that if they smoke, the child will have a markedly increased risk of SIDS due to the smoking.
This study has several important limitations related to design and the nature of the data set used. Given the rich nature of the Colorado Birth Registry, we controlled for many potential confounding variables, but others may exist. This analysis likely represents an under-estimate of the actual associated risk with prenatal smoking, given that it relies on maternal report of a behavior widely known to be harmful. We suspect that self-report of maternal smoking underestimates the true rate of maternal smoking. Fetal and infant exposure to tobacco smoke occurs in manner ways, including prenatal maternal smoking, prenatal maternal second-hand smoke exposure, and postnatal smoke exposure by one or more care providers in the home. In this study we cannot distinguish among these exposure pathways, which may result in an erroneous under or over estimate of risk. The time period spanned by this study includes the rollout of the Back to Sleep campaign in the United States and our dataset did not assess whether parents positioned their infants prone or supine. We therefore could not control for this ecological association with SIDS. The very nature of retrospective data creates limitations in that we cannot carefully control the setting in which the data are collected, or the individual collecting the data. Missing data records are a third limitation, although this appears to have not been a major issue with this dataset.