Preschoolers are more likely to wheeze and develop asthma when their mothers smoked early in pregnancy.
A European analysis of more than 21,000 children found the more a woman smoked during her first trimester of pregnancy, the greater the risk her children would develop breathing problems before kindergarten.
The link was specific to smoking early in pregnancy -- when a woman might not know she's pregnant.
Limitations of the study include the use of parental questionnaires, not official diagnoses, to gather information.
