Maternal smoking during pregnancy and type 1 diabetes in the offspring: a nationwide register-based study with family-based designs

Table 2 Associations^a between maternal smoking during pregnancy and offspring type 1 diabetes in different study designs

Smoking status	Non-smoking	1–9 cigarettes/day	≥ 10 cigarettes/day	P^†
Cohort analysis
No. of cases	16,084	1684	849
Person years	38,819,013	5,235,610	2,863,183
Incidence (no./10,000 person years)	4.1	3.2	3.0
Model 1	1.00	0.80 (0.76–0.85)	0.75 (0.70–0.80)	0.100
Model 2	1.00	0.80 (0.76–0.84)	0.74 (0.69–0.80)	0.076
Model 3	1.00	0.80 (0.76–0.85)	0.74 (0.69–0.80)	0.066
Cousin comparison analysis
No. of cases	6765	801	422
Model 1	1.00	0.75 (0.68–0.83)	0.69 (0.60–0.79)	0.245
Model 2	1.00	0.75 (0.68–0.83)	0.67 (0.58–0.77)	0.114
Model 3	1.00	0.75 (0.68–0.83)	0.66 (0.58–0.76)	0.111
Sibling comparison analysis
No. of cases	12,483	1193	608
Model 1	1.00	0.79 (0.71–0.89)	0.70 (0.60–0.83)	0.116
Model 2	1.00	0.80 (0.71–0.90)	0.71 (0.60–0.84)	0.119
Model 3	1.00	0.80 (0.71–0.90)	0.71 (0.60–0.84)	0.115

Model 1 was adjusted for sex and year of birth. The Cox model in the cohort analysis was fitted with a gamma-frailty component to account for the clustering of siblings from the same mother. The conditional logistic model in the sibling (or cousin) comparison analysis conditioned on sibling (or cousin) groups
Model 2 was additionally adjusted for maternal education, paternal education, maternal age at delivery, maternal body mass index during pregnancy, maternal history of diabetes, paternal history of diabetes, and birth order based on model 1
Model 3 was additionally adjusted for gestational age and birth weight for gestational age based on model 2
^†The P value indicted whether there was a difference in the incidence of type 1 diabetes between maternal smoking 1–9 cigarettes/day and ≥ 10 cigarettes/day
^aHazard ratios (95% confidence intervals) are presented for the cohort analysis, and odds ratios (95% confidence intervals) are presented for the cousin and sibling comparison analyses

ISSN: 1741-7015