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NEW YORK (Reuters Health) - 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," Dr. Kristen Stone of Women and Infants Hospital in Providence, Rhode Island, one of the study's authors, told Reuters Health. What's more, while most of the women who smoked during pregnancy used at least one other drug, Stone and her team found that nicotine was the only substance associated with sleep problems.Stone and colleagues from centers in Miami, Detroit, and Memphis are following nearly 1,400 children born in 1993, 1994 or 1995 to investigate the long-term effects of exposure to substances during pregnancy. The current study included children for whom data was available up to age 12. Among the 808 study participants, 374 had been exposed to cocaine or opiates such as heroin before birth, while 434 had not.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. Being exposed to cocaine, opiates, marijuana, or alcohol in the womb had no effect on a child's risk of having sleep problems, but nicotine did, 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.The findings shouldn't be seen as showing that prenatal use of alcohol and drugs aside from cigarettes isn't as harmful to a child's sleep as smoking in pregnancy, Stone noted. Cigarettes are different from other substances, she explained, in that a person who smokes will typically do so much more frequently than a drug abuser uses cocaine or opiates. Further, she said, many of the mothers in the study were using multiple substances while they were pregnancy. "When those substances are inside of us at the same time, they basically become a whole new substance because of their interactions with each other," she added. All of this makes it difficult to tease out the effects of nicotine and other drugs, according to the researcher.In an editorial accompanying the study, Drs. Gideon Koren and Irena Nulman of the Hospital for Sick Children, Toronto, note that smoking mothers are different from nonsmoking mothers in many ways, and that 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. When a child does have sleep problems, Stone said, "early and careful attention" to these issues can go along way toward helping that child sleep better."Even an emphasis on basic behavioral sleep education could serve those children well," she added. "Doing that would then likely improve the daytime experience for those children as well."