Of the 322 323 children studied, 3388 were diagnosed with ASD, including 2963 unexposed, 115 exposed to preexisting maternal type 2 diabetes, and 310 exposed to gestational diabetes. The unadjusted incidences were 1.77, 3.26, and 2.14 per 1000, respectively. More than 99% of infants who were exposed to maternal diabetes in utero did not develop ASD. However, in adjusted analyses, the authors found an increased risk in the subgroup of children exposed to gestational diabetes at 26 weeks or earlier. The hazard ratio for preexisting type 2 diabetes was 1.21 (95% CI, 0.97-1.52) and for gestational diabetes at 26 weeks or earlier, 1.42 (95% CI, 1.15-1.74). Thus, the timing for this environmental exposure is isolated to early pregnancy.