No significant differences in risks were found to be associated with maternal or paternal smoking at any time (odds ratio (OR) = 0.92 for mothers and 1.06 for fathers), during the year of birth of the child (which included both the prenatal and postnatal periods) (ORs = 0.84 for<1 pack/day and 1.0 for>or = 1 pack/day for mothers, and 0.68 for<1 pack/day and 1.07 for>or = 1 pack/day for fathers), or 2 years before the child was born, i.e., the pre-conception period (ORs = 0.75 for<1 pack/day and 1.01 for>or = 1 pack/day for mothers, and 0.90 for<1 pack/day and 1.15 for>or = 1 pack/day for fathers). Mothers were also specifically asked if they smoked during the pregnancy, and no association was found compared with never smokers (OR = 1.08, 95% confidence interval (CI) 0.80-1.45) or for ever-smokers who continued to smoke during pregnancy compared with those who stopped smoking during pregnancy (OR = 1.15, 95% CI 0.75-1.78).