Secondhand smoke exposure and risk of wheeze in early childhood: a prospective pregnancy birth cohort study

Background: Evidence regarding the independent and additive effects of both pre- and postnatal smoking exposure on the risk of wheeze in children is limited. The purpose of this prospective pregnancy birth cohort study was to examine the association between prenatal and postnatal tobacco smoke exposure during the first year of life and the risk of wheeze in Japanese children aged 23 to 29 months. Methods: Study subjects were 1354 Japanese mother-child pairs. Information on the variables under study was obtained using repeated questionnaires that were completed by mothers, first prior to delivery, then shortly after birth and subsequently around 4, 12, and 24 months after delivery. Wheeze was defined according to the criteria of the International Study of Asthma and Allergies in Childhood. Results: Compared with no maternal smoking during pregnancy, maternal smoking throughout pregnancy was significantly associated with an increased risk of wheeze in children, yet there were no associations between maternal smoking in the first trimester only or in the second and/or third trimesters and the risk of wheeze. No association was observed between postnatally living with at least one household smoker and the risk of wheeze. An analysis to assess the additive effect of prenatal and postnatal smoking exposure revealed that, compared with children not exposed to maternal smoking during pregnancy and not postnatally living with at least one household smoker, those who were both exposed to maternal smoking during pregnancy and postnatally living with at least one household smoker had twofold odds of developing wheeze. Conclusions: Our findings suggest that maternal smoking throughout pregnancy might be associated with an increased risk of wheeze in children. There is also the possibility of a positive additive effect of pre- and postnatal smoking exposure on the risk of childhood wheeze.