Smoking during pregnancy and infant death

Smoking has been shown to be the strongest modifiable prenatal risk factor for sudden and unexpected infant death (SUID) in industrialized nations, but studies in this area are limited in scope. New research in Pediatrics attempts to quantify the effects of maternal smoking, smoking cessation, and smoking reduction in pregnancy on SUID rates. 

Using data from the Centers for Disease Control and Prevention Birth Cohort Linked Birth/Infant Death Data Set, the authors looked at nearly 20.7 million births and 19,000 SUIDs between 2007 and 2011. Of the births, nearly 12.5 million had complete prenatal smoking information and 10,737 met the study’s SUID definition (infant death with ICD-10 codes: R65 (sudden infant death syndrome [SIDS]), R99 (ill-defined and unknown cause), or W75 (accidental suffocation or strangulation in bed).

In regard to maternal smoking frequency, the authors found that in 2011, 11.5% of mothers smoked in the 3 months before pregnancy and 8.9% smoked during pregnancy. The authors noted that 24.3% of smokers who did so before conception quit before the first trimester. 

A positive correlation between the average number of daily cigarettes and risk of SUID was apparent. When comparing non-smoking and smoking 1 cigarette daily throughout pregnancy, there was a twofold-increased SUID risk (aOR =1.98; 95% CI, 1.73-2.28). For smokers who consumed 1 to 20 cigarettes daily, risk of SUID increased linearly with each additional cigarette smoked per day. However, risk plateaued after 20+ cigarettes per day, though the authors note that this flattening could be a result of a lower number of cases.

SUID risk was progressively higher in mothers who smoked before pregnancy and quit before pregnancy (aOR = 1.47; 95% CI 1.16- 1.87), those who did not smoke before but smoked during pregnancy (aOR = 2.22; 95% CI 1.15-4.29) and those who smoked both before and during pregnancy (aOR = 2.52; 95% CI 2.25 – 2.83), compared with mothers who did not smoke in the 3 months before or during pregnancy.

If a causal relationship is assumed, the authors said, 22% of SUIDs can be attributed to maternal smoking during pregnancy. They believe their findings support a need for increased education about the importance of smoking cessation before conception. If expectant mothers are unable to completely quit smoking, the data also underscore a need to reduce the average number of cigarettes daily, at the very least. Ultimately, though, SUID rates in the United States could be reduced dramatically if no women smoked while pregnant.

“We found that smoking just a single cigarette a day on average during pregnancy doubled the risk of a sudden unexpected infant death (SUID),” Tatiana Anderson, MD,a researcher in Seattle Children's Center for Integrative Brain Research and lead author on the studytold Contemporary OB/GYN.“Between 1-20 cigarettes, the risk was linear, such that smoking a pack a day (20 cigarettes) led to a three-fold risk.  Smoking in the three months before pregnancy and quitting by the 1st trimester still led to a nearly 50% increased risk of SUID. In the US, 8.9% of mothers smoke during pregnancy, of which 55% do not quit or reduce their cigarette consumption.” 

Dr. Anderson continued, “We believe that it is important for doctors to encourage their patients to quit smoking well before they plan to become pregnant to best minimize the risk of SUID.  For those that are unable to quit, however, every cigarette that can be reduced measurably decreases the risk of a sudden infant death.  We estimate that 800 infant deaths in the US could be prevented annually if no women smoked during pregnancy.”