In Table 1, maternal characteristics and obstetric outcomes are presented. The analyses show that maternal age and BMI were higher in both the elective and acute CS groups, compared with the vaginal delivery group (p < 0.001; Table 1).
In Fig. 1, the proportion of mothers with and without diabetes who underwent CS is shown. Delivery by CS was more common in mothers with diabetes than in mothers without diabetes: 78.8% (476/604) vs 12.7% (5,873/46,276) (p < 0.001).
Table 2 shows in more detail that mothers without diabetes had the same proportion of children who later developed type 1 diabetes, whether they were delivered vaginally or by CS. Mothers with diabetes had more children who later developed type 1 diabetes no matter what mode of delivery was used, and the majority of mothers with type 1 diabetes delivered by CS.
In mothers of children who developed type 1 diabetes, 14.7% gave birth by CS compared with 13.3% of mothers of children in the control group (Table 3; p < 0.001). Furthermore, 4.8% (n = 453) of children who developed type 1 diabetes were delivered by acute CS compared with 3.3% (n = 1,254) of controls (Table 3; p < 0.001). There was no difference between mothers with children who later developed diabetes and mothers of control children regarding planned CS (9.9% each). However, 0.9% (n = 328) of control children had a mother with diabetes compared with 2.9% (n = 276) of children who later developed type 1 diabetes (p < 0.001). In addition, we also know that 213 of the 604 mothers with diabetes had type 1 diabetes; of these, 73 were mothers to control children (0.4%) and 140 were mothers (3.2%) to children who later developed diabetes (Table 3; p < 0.001). Overall, and in both groups, mothers diagnosed with diabetes had significantly higher BMI in early pregnancy compared with mothers who did not have diabetes (Table 3; p < 0.001).
Unexpectedly, for children of mothers with type 1 diabetes, we observed a higher degree of development of type 1 diabetes among girls than boys (3.5% vs 3.0%). Adding mothers with all types of diabetes made the figures more equal: 3.1% for girls and 2.8% for boys.
In the extended analyses of type 1 diabetes cases and four matched controls per case of the relationship between the development of diabetes among the children and the presence of maternal diabetes, it was found that boys more often developed type 1 diabetes if the mother had diabetes, compared with boys born to mothers without diabetes (47.4% (144/304) vs 19.7% (5,028/25,556), p < 0.001). The figures were about the same for girls (44% [132/300] vs 19.7% [4,072/20,720], p < 0.001). When restricting our comparisons to mothers with and without type 1 diabetes this pattern was strengthened. In boys, if the mother had type 1 diabetes at the time of delivery, 61.3% (68/111) developed type 1 diabetes compared with 19.5% (2,182/11,139) if the mother did not have type 1 diabetes, p < 0.001. Corresponding figures for girls were 70.6% (72/102) and 19.5% (1,989/10,203), respectively, p < 0.001.
If the mother had diabetes (either type 1, type 2 or gestational), 42.1% (53/126) of the children delivered vaginally developed type 1 diabetes (Table 2); of those delivered with acute CS 52.3% (91/174) developed type 1 diabetes; and of those delivered by planned CS 43.4% (132/304) developed type 1 diabetes. Corresponding figures for children who developed diabetes but had mothers without diabetes were 19.7% (n = 7,946), 18.6% (n = 833) and 22.9% (n = 321), respectively (p < 0.001). Furthermore, only including mothers with type 1 diabetes (n = 213) in the analysis, 97.7% of the children were found to have been delivered by CS, of which 62% (n = 129) were by planned CS. In comparison, among mothers without type 1 diabetes (n = 21,342), 17.1% of children had been delivered by CS (p < 0.001), of which 36.3% (n = 1,323) were by planned CS.
Mothers of children who did not develop type 1 diabetes had about the same mean age as mothers of children who later developed type 1 diabetes: 29.02 ± 5.1 and 29.16 ± 5.1 years. However, mothers who delivered by CS were significantly older than mothers who had a vaginal delivery: 30.5 ± 5.2 and 28.8 ± 5.0 years (p < 0.001). Moreover, mothers who had diabetes (n = 604) were older than mothers without diabetes (n = 46,276): 30.55 ± 5.2 and 29.03 ± 5.1 years (p < 0.001). In mothers with diabetes, there were also a higher proportion of mothers who were above 30 years of age when giving birth: 58.9% compared with 45.1% of mothers without diabetes (p < 0.001).
The multivariate analysis
In the multivariate analysis, the OR for developing type 1 diabetes, when only including CS in the logistic regression model, was estimated to be 1.12 (95% CI 1.06, 1.2; p < 0.001), but when only including maternal diabetes the OR increased to 3.44 (95% CI 2.92, 4.04; p < 0.001) and when only including maternal type 1 diabetes the OR was 7.9 (95% CI 5.94, 10.5; p < 0.001). Including both maternal diabetes and CS in the model the OR for CS was no longer statistically significant while maternal diabetes remained statistically significant (OR 2.89, 95% CI 2.03, 4.1; p < 0.001). Including maternal age in the multiple logistic regression model did not alter these findings (Table 4). An interaction term between maternal diabetes and CS also did not alter these findings, as it was statistically insignificant.
The diabetes population
Table 5 shows the OR for CS in this population of children who later developed type 1 diabetes. For instance, when only including maternal age the OR was 1.06. If the mother had diabetes at the time of delivery the OR was 28.9. The adjusted values are nearly the same as the unadjusted ones. Hence, if a mother of a child who later developed type 1 diabetes had type 1 diabetes at the time of delivery this child had the highest likelihood of having been delivered by CS. The OR for a child who later developed type 1 diabetes to be delivered by CS if the mother had no diabetes at the time of delivery was 1.0 (95% CI 0.93, 1.07).