Table 3. Associations of environmental tobacco smoke exposure during the first 6 months of life and childhood-onset type 1 diabetes in the Norwegian Mother and Child Cohort Study (MoBa) and the Danish National Birth Cohort (DNBC).

Study	Maternal smoking at child age 0-6 months	Person years	Events	Adjusted a HR (95% CI)	Heterogeneity P-value
MoBa	No	871,320	311	1
	Yes	155,707	38	0.70 (0.42, 1.2)
DNBC	No	1,002,728	287	1
	Yes	207,091	53	0.86 (0.53, 1.4)
Combined	No	1,874,048	598	1	0.77
	Yes	362,798	91	0.78 (0.55, 1.1)
Study	Household smoking at child 0-6 months	Person years	Events	Adjusted b HR (95% CI)	Heterogeneity P-value
MoBa	No	989,965	334	1
	Yes	37,062	15	1.1 (0.64, 2.0)
DNBC	No	1,050,488	298	1
	Yes	159,331	42	1.2 (0.78, 1.7)
Combined	No	2,040,453	632	1
	Yes	196,393	57	1.2 (0.83, 1.6)	0.94

CI, confidence interval; HR, hazard ratio.

^aAdjusted for maternal smoking during pregnancy, age, parity, education, diabetes (maternal type 1 diabetes in MoBa and all types of maternal diabetes in DNBC), and pre-pregnancy BMI.

^bAdjusted for parental (both maternal and paternal) age, education, diabetes (maternal type 1 diabetes in MoBa and all types of maternal diabetes in DNBC), BMI and smoking during pregnancy, in addition to maternal parity.

We imputed 20 datasets of missing exposure and covariate information by multiple imputation by chained equations, and run analyses with these. This analysis takes into account uncertainty in the imputation