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Background Conflicting results have been reported concerning possible adverse effects on the cognitive function of offspring of mothers with type 1 diabetes (O-mT1D). Previous studies have included offspring of parents from the background population (O-BP), but not offspring of fathers with type 1 diabetes (O-fT1D) as the unexposed reference group. Methods and findings This is a population-based retrospective cohort study from 2010 to 2016. Nationally standardized school test scores (range, 1 to 100) were obtained for public school grades 2, 3, 4, 6, and 8 in O-mT1D and compared with those in O-fT1D and O-BP. Of the 622,073 included children, 2,144 were O-mT1D, and 3,474 were O-fT1D. Multiple linear regression models were used to compare outcomes, including the covariates offspring with type 1 diabetes, parity, number of siblings, offspring sex, smoking during pregnancy, parental age, and socioeconomic factors. Mean test scores were 54.2 (standard deviation, SD 24.8) in O-mT1D, 54.4 (SD 24.8) in O-fT1D, and 56.4 (SD 24.7) in O-BP. In adjusted analyses, the mean differences in test scores were −1.59 (95% CI −2.48 to −0.71, p < 0.001) between O-mT1D and O-BP and −0.78 (95% CI −1.48 to −0.08, p = 0.03) between O-fT1D and O-BP. No significant difference in the adjusted mean test scores was found between O-mT1D and O-fT1D (p = 0.16). The study’s limitation was no access to measures of glycemic control during pregnancy. Conclusions O-mT1D achieved lower test scores than O-BP but similar test scores compared with O-fT1D. Glycemic control during pregnancy is essential to prevent various adverse pregnancy outcomes in women with type 1 diabetes. However, the present study reduces previous concerns regarding adverse effects of in utero hyperglycemia on offspring cognitive function. Anne Lærke Spangmose and colleagues examine the association between school performance and exposure to maternal or paternal type 1 diabetes in utero in Denmark. Author summary Why was this study done? The influence of maternal diabetes during pregnancy on offspring cognition has been widely explored because high blood sugar levels in pregnant women are suspected to affect fetal development, including the brain. Limited evidence is available on how different subtypes of maternal diabetes are associated with offspring cognition (e.g., gestational diabetes and type 1 and type 2 diabetes). Limited data are available on the association between maternal type 1 diabetes during pregnancy and offspring cognition using offspring of fathers with type 1 diabetes (O-fT1D) as the unexposed reference group, which allows for adjustments of potentially shared genes and familial stress of having a parent suffering from a serious chronic disease like diabetes. What did the researchers do and find? Data were identified from Danish registers, and test scores in math (grades 3 and 6) and reading (grades 2, 4, 6, and 8) were obtained on all Danish children attending public schools from 2010 to 2016. We included 2,144 offspring of mothers and 3,474 O-fT1D and 616,455 from the background population, including 1,704,447 test scores. Offspring of mothers with type 1 diabetes (O-mT1D) achieved lower test scores than offspring in the background population but similar test scores compared with O-fT1D. What do these findings mean? The lower test scores in the O-mT1D appear to reflect a negative association of having a parent with type 1 diabetes rather than a specific adverse effect of maternal type 1 diabetes during pregnancy on the fetus. This study presents evidence of an alternative explanation for the previously observed adverse effect of maternal type 1 diabetes during pregnancy on offspring cognitive development.