Background: Maternal asthma has been proposed as an early life risk factor for type 1 diabetes, but it remains unclear if the association is due to offspring exposure to maternal asthma specifically during pregnancy or familial confounding by shared genes or environment. Aims and objectives: To investigate the association between maternal asthma during pregnancy and to understand the role of familial confounding by comparing to pre-existing maternal asthma before pregnancy and by using paternal asthma as a negative control. Methods: In this sample of the Swedish population, 1.4 million offspring born 2007?2020 were linked to their biological parents and followed with information from national registers from preconception, during pregnancy, and childhood, through 2021. Asthma and type 1 diabetes were ascertained from validated combinations of diagnoses and medication prescriptions. Associations were estimated using Cox regression analysis adjusted for confounders. Results: Offspring exposed to maternal asthma during pregnancy (2.5%) did not have an increased risk of type 1 diabetes compared to those unexposed (HR 0.90 [95% CI 0.75, 1.08]). However, pre-existing maternal asthma (11.9%) and paternal asthma (9.0%) were both associated with offspring type 1 diabetes risk (1.18 [1.08, 1.29] and 1.21 [1.10, 1.34] respectively). Conclusions: Maternal asthma is a risk factor for offspring type 1 diabetes, but not specifically during pregnancy. Since maternal exposure occurring before pregnancy or from the father cannot entail a direct effect in utero on the unborn child, our findings support evidence for familial confounding between asthma and type 1 diabetes.