Smoking during pregnancy is associated with wheeze and asthma in preschool children, a new study has found.
The conditions also occured where the child was not exposed to maternal smoking in late pregnancy or after birth.
The study, led by Dr Åsa Neuman of Karolinska Institutet in Stockholm, reviewed data on 21,600 children, including 735 who were exposed to maternal smoking only during pregnancy.
"Epidemiological evidence suggests that exposure to maternal smoking during fetal and early life increases the risk of childhood wheezing and asthma, but earlier studies were not able to differentiate the effects of prenatal and postnatal exposure," said Dr Neuman.
The study adjusted the data for sex, parental education, parental asthma, birth weight and siblings, and found that maternal smoking only during pregnancy was associated with increased risks for wheeze and asthma between the ages of four and six.
"These children were at increased risk for wheeze and asthma at preschool age. 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," he added.
"These results indicate that the harmful effects of maternal smoking on the fetal respiratory system begin early in pregnancy, perhaps before the woman is even aware that she is pregnant," Dr Neuman said.
"Teens and young women should be encouraged to quit smoking before getting pregnant," Dr Neuman said.
The findings are published online in the American Thoracic Society's American Journal of Respiratory and Critical Care Medicine.