A study in the April 2019 Pediatrics found that infants whose mothers smoked during pregnancy had more than twice the risk of dying suddenly and unexpectedly.

For the study, “Maternal Smoking Before and During Pregnancy and the Risk of Sudden Unexpected Infant Death,” published online March 11, researchers analyzed vital statistics data for all U.S. live births from 2007 to 2011 with complete smoking information — about 20 million births.

They included 19,127 deaths attributed to three major categories of Sudden Unexplained Infant Death (SUID): Sudden Infant Death Syndrome; ill-defined and unknown causes of death; and accidental suffocation and strangulation in bed under 1 year of age.

They found the largest predictor of SUID risk with maternal prenatal smoking was the average number of cigarettes smoked daily during pregnancy. Among pregnant women who smoked 1 to 20 cigarettes per day, the odds of SUID increased by 0.07 with each additional cigarette. The analysis also showed that while most smokers (55 percent) did not quit or cut back during pregnancy, those who did saw a measurable reduction in risk.

Women who reduced their smoking by the third trimester saw a 12-percent decrease in SUID risk; quitting entirely by third trimester, which 20 percent of the women did, was associated with a 23-percent reduction in risk.

The researchers conclude that smoking cessation efforts before and during pregnancy are key to decreasing SUID risk, and that pregnant women unable to quit entirely should be advised to reduce the amount they smoke.

They estimate that U.S. SUID rates could be reduced by 22 percent, preventing about 800 infant deaths each year, if no women smoked during pregnancy.