Mothers who smoke during pregnancy are more likely to have children with sleep problems from birth all the way through age 12, new research shows.
"The more cigarettes that mothers smoked during pregnancy, the more sleep problems the children had," said Dr. Kristen Stone of Women and Infants Hospital in Providence, Rhode Island, USA, who was one of the study's authors.
Most of the women who smoked during pregnancy used at least one other drug, but Dr. Stone and her colleagues found that nicotine was the only substance associated with sleep problems.
The multicentre team is following nearly 1,400 children born in 1993, 1994 or 1995 to investigate the long-term effects of in utero exposure to cocaine, opiates, marijuana, alcohol, and/or nicotine.
The current study, reported in the May issue of Archives of Pediatrics and Adolescent Medicine, included 808 children for whom data was available up to age 12, including 374 who had been exposed to cocaine or opiates such as heroin before birth.
The children's mothers or other caregivers reported on whether a child had difficulty falling asleep and staying asleep during three periods: one month to four years of age; five to eight years, and nine to 12 years. In utero exposure to cocaine, opiates, marijuana, or alcohol had no effect on a child's risk of sleep problems.
"Of the 5 substances, prenatal nicotine exposure was the only unique predictor of sleep problems," the authors report -- and problems were seen at each of the three time points.
The researchers do not report what percentage of children had sleeping problems, but used a common measure of such problems that assigns points for items such as talking in one's sleep, sleepwalking, and having trouble falling asleep.
The link remained even after the researchers took into account factors such as socioeconomic status, whether or not a child had been abused, and whether the mother or caregiver smoked after the child was born.
Dr. Stone acknowledges that many mothers in the study were using multiple substances while they were pregnant, which makes it difficult to tease out the effects of nicotine and other drugs.
In an editorial, Drs. Gideon Koren and Irena Nulman of the Hospital for Sick Children in Toronto note that smoking mothers are different from nonsmoking mothers in many ways, and that Dr. Stone and her team were unable to account for all of them.
Until it's possible to identify all these factors and use statistical techniques to adjust for them, they add, it would be "premature" to say that cigarette smoke exposure in the womb caused a child's sleep problems.