FRIDAY, Aug. 17, 2012 (HealthDay News) -- New research from Sweden suggests that smoking during early pregnancy may boost the risk that preschool children will develop asthma and wheezing problems, even if the kids aren't exposed to smoke after birth.
The findings were published online Aug. 17 in the American Journal of Respiratory and Critical Care Medicine.
Previous research has suggested that a mother who smokes -- both during and after pregnancy -- boosts a child's risk of wheezing and asthma, study author Dr. Asa Neuman, at the Institute of Environmental Medicine at the Karolinska Institute, in Stockholm, said in a journal news release.
"Our study, a large pooled analysis of eight birth [groups] with data on more than 21,000 children, included 735 children who were exposed to maternal smoking only during pregnancy," Neuman said.
"These children were at increased risk for wheeze and asthma at preschool age," Neuman said. "Furthermore, the likelihood of developing wheeze and asthma increased in a significant dose-response pattern in relation to maternal cigarette consumption during the first trimester."
The risk of wheezing and asthma rose even after researchers adjusted their statistics so they wouldn't be thrown off by factors such as gender, birth weight and the education levels of parents. The study relied on parent questionnaires to figure out whether the kids suffered from asthma and wheezing.
Maternal smoking seemed to be riskiest during the first trimester of pregnancy. Smoking by the mother in the third trimester or the first year of life didn't boost the risk of the conditions.
"These results indicate that the harmful effects of maternal smoking on the fetal respiratory system begin early in pregnancy, perhaps before the women is even aware that she is pregnant," Neuman said.
The study, however, doesn't prove a cause-and-effect relationship, as the information came from parental responses rather than a clinical study of children whose mothers who did or did not smoke while pregnant.
For more on smoking, try the U.S. National Library of Medicine.