Open Access

Justine Tanoey, Christina Baechle, Hermann Brenner, Andreas Deckert, Julia Fricke, Kathrin Günther, André Karch, Thomas Keil, Alexander Kluttig, Michael Leitzmann, Rafael Mikolajczyk, Nadia Obi, Tobias Pischon, Tamara Schikowski, Sabine M. Schipf, Matthias B. Schulze, Anja Sedlmeier, Ilais Moreno Velásquez, Katharina S. Weber, Henry Völzke, Wolfgang Ahrens, Sylvia Gastell, Bernd Holleczek, Karl-Heinz Jöckel, Verena Katzke, Wolfgang Lieb, Karin B. Michels, Börge Schmidt, Henning Teismann, Heiko Becher

• (1) Background: Global incidence of type 1 diabetes (T1D) is rising and nearly half occurred in adults. However, it is unclear if certain early-life childhood T1D risk factors were also associated with adult-onset T1D. This study aimed to assess associations between birth order, delivery mode or daycare attendance and type 1 diabetes (T1D) risk in a population-based cohort and whether these were similar for childhood- and adult-onset T1D (cut-off age 15); (2) Methods: Data were obtained from the German National Cohort (NAKO Gesundheitsstudie) baseline assessment. Self-reported diabetes was classified as T1D if: diagnosis age ≤ 40 years and has been receiving insulin treatment since less than one year after diagnosis. Cox regression was applied for T1D risk analysis; (3) Results: Analyses included 101,411 participants (100 childhood- and 271 adult-onset T1D cases). Compared to “only-children”, HRs for second- or later-born individuals were 0.70 (95% CI = 0.50–0.96) and 0.65 (95% CI = 0.45–0.94), respectively, regardless of parental diabetes, migration background, birth year and perinatal factors. In further analyses, higher birth order reduced T1D risk in children and adults born in recent decades. Caesarean section and daycare attendance showed no clear associations with T1D risk; (4) Conclusions: Birth order should be considered in both children and adults’ T1D risk assessment for early detection.