Mothers who smoke during pregnancy are more than twice as likely to have babies die suddenly in their sleep as women who avoid tobacco, a US study suggests.
More than 3,700 US infants up to 12 months old die each year of sleep-related causes like sudden infant death syndrome (SIDS) or accidental suffocation and strangulation in bed or other unknown causes, researchers report in Pediatrics.
Smoking has long been linked to an increased risk of these fatalities, known as sudden unexpected infant death (SUID), but the current study offers fresh evidence of how much cutting back or quitting might help improve babies' survival odds.
"We found that smoking even a single cigarette daily during pregnancy doubled the risk of SUID," said lead study author Tatiana Anderson of the Seattle Children's Research Institute.
Anderson's team examined data from the US Centres for Disease Control and Prevention on more than 20 million births and more than 19,000 SUID cases from 2007 to 2011. For more than 12.4 million of those births and nearly 11,000 SUID cases, the researchers had prenatal smoking information.
"Mothers who smoked in the three months before pregnancy and quit by the first trimester still had a nearly 50 per cent greater chance of a SUID death compared to nonsmokers," Anderson said by email.
Still, smokers who cut back during pregnancy had a 12 per cent lower risk of SUID than smokers who didn't curb their cigarette use, the researchers found. Smokers who quit had a 23 per cent lower risk of SUID.
For women who did smoke during pregnancy, each additional daily cigarette from 1 to 20 increased the odds of SUID by seven per cent.
Approximately 22 per cent of SUID cases in the US may be directly attributed to mothers smoking during pregnancy, researchers estimated.
The study wasn't a controlled experiment designed to prove whether or how smoking causes SUID. Furthermore, while the effects of smoking during pregnancy were derived from the entire cohort, the effects of smoking before pregnancy were determined using only data from 2011 (3.1 million total births and 2585 SUIDs).
Researchers also lacked data on smoking by fathers or other adults around pregnant women and newborns, which means the study may have underestimated babies' exposure to tobacco.
"Since many women who smoke during pregnancy continue to smoke after they deliver, it is a little difficult to separate out the effects of prenatal and postnatal cigarette smoke exposure," said Dr. Michael Goodstein, division chief of WellSpan Health Neonatology in York, Pennsylvania.
"But there are many studies that have associated postnatal smoke exposure with an increased risk of SIDS," Goodstein, who wasn't involved in the study, said by email. Smoking by mothers, fathers and anyone else around pregnant women and babies may increase not just the risk of SUID but also childhood asthma and respiratory illnesses, Goodstein added.
Both parents should quit smoking before trying to conceive, said Michael Gradisar, a psychology researcher at Flinders University in Australia who wasn't involved in the study.
"Babies exposed to smoking during pregnancy develop abnormalities in important areas of the brain that regulate breathing and temperature," Gradisar said by email. "When these infants sleep, these basic functions may be compromised resulting in SIDS."
To prevent SUID, doctors advise parents to put babies to sleep on their backs without blankets or other soft bedding and toys that could pose a suffocation risk.
Babies exposed to smoke in the womb or early in life may be especially vulnerable to these hazards, said Anna Pease, a researcher at the University of Bristol in the UK who wasn't involved in the study.
"All babies have an arousal, or wake up, system that triggers if they don't have enough air around the face, for example if covered with a blanket," Pease said by email. "In babies that have been smoke exposed, their wake up system does not trigger as soon as it should and we think this could be partly why these babies are more likely to die (of) SIDS."