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Background: Previous studies have suggested that fetal smoke exposure is associated with increased risks of wheezing during childhood. The underlying pathways are unknown. We examined the associations of parental smoking during pregnancy with wheezing in preschool children and whether these associations are explained by postnatal smoke exposure or small for gestational age at birth.Methods: This study was embedded in a population-based prospective cohort study. Parental smoking was prospectively assessed by questionnaires. Wheezing was reported at 1 to 4 years. Small for gestational age at birth was available from registries. The analyses were based on 4,574 subjects.Results: Maternal smoking during the first trimester only was not associated with wheezing. Continued maternal smoking in pregnancy was associated with the risk of wheezing at 1 to 4 years (P for trends < .05). The strongest effect estimates were observed for frequent wheezing (four or more episodes of wheezing per year) until age 3 years (OR [95% CI]: age 1,1.64 [1.12-2.40]; age 2, 1.64 [1.01-2.64]; age 3, 2.19 [1.24-3.86]). Among children of nonsmoking mothers, fetal exposure to paternal smoking was not consistently associated with the risks of wheezing. The associations of continued maternal smoking during pregnancy with wheezing symptoms were independent of postnatal smoke exposure or small for gestational age at birth.Conclusions: Fetal exposure to continued maternal smoking is associated with increased risks of wheezing in preschool children. Further research is needed to explore the effects of paternal smoking. Diminishing maternal smoking before conception or in early pregnancy is likely to have the greatest impact on reducing childhood wheezing. [ABSTRACT FROM AUTHOR]