AIMS To determine whether maternal smoking during pregnancy is a risk factor for reported wheeze in early childhood that is independent of postnatal environmental tobacco smoke (ETS) exposure and other known risk factors.

RESULTS A total of 1869 (21.8%) children had reported wheeze between 18 and 30 months of age, and 3496 (40.8%) had reported wheeze in one or more of the three study periods (birth to 6 months, 6–18 months, 18–30 months). The risk of wheeze between 18 and 30 months of age was higher if the mother smoked during pregnancy. This relation did not show a dose–response effect and became less obvious after adjustment for the effects of other factors. Average daily duration of ETS exposure reported at 6 months of age showed a dose–response effect and conferred a similar risk of reported wheeze. Factors associated with early childhood wheeze had the following adjusted odds ratios: maternal history of asthma 2.03 (1.74 to 2.37); preterm delivery 1.66 (1.30 to 2.13); male sex 1.42 (1.28 to 1.59); rented accommodation 1.29 (1.11 to 1.51); and each additional child in household 1.13 (1.04 to 1.24).

CONCLUSIONS Maternal smoking during pregnancy may be a risk factor for reported wheeze during early childhood that is independent of postnatal ETS exposure. For wheeze between 18 and 30 months of age, light smoking during the third trimester of pregnancy appears to confer the same risk as heavier smoking.