This study includes children from the Air Pollution and Childhood Cancers (APCC) study previously described [36 (link)]. Childhood cancer cases aged 5 years or younger at diagnosis were identified from the California Cancer Registry from 1988–2013. This analysis was restricted to young children as we hypothesized that pregnancy exposures are likely to be more relevant to the etiology of cancers diagnosed in early childhood. Approximately 89% of cases were successfully matched to their birth certificate by first and last name, date of birth, and when available, social security number. It is likely that children we were unable to match were those who moved to California after birth but before the age of 6 years [37 (link)]. Controls were frequency-matched by year of birth to all childhood cancer cases during the study period (20:1 matching rate) and randomly selected from all California birth certificates. The rationale for choosing a 20:1 ratio was to ensure that in the APCC study, a study of environmental exposures, there would be sufficient controls selected who resided in rural areas. Selection criteria for controls consisted of absence of a cancer diagnosis before 6 years of age in California. Also, potential control children were excluded if they died of any cause prior to age 6 (n=1,792). We also excluded children that were missing sex (n=3), births that were likely not viable (gestational age <20 weeks and/or birthweight <500g) (n=169), and children diagnosed with Down syndrome (n=151). The latter was done because Down syndrome is a strong risk factor for childhood cancer [3 (link)] and potentially related to pregnancy-related characteristics, including maternal obesity [38 (link)]. Additionally, mothers who had extreme or implausible BMI values (<17 kg/m2 or > 45 kg/m2) and gestational weight gain values (< −2 kg or >32 kg) were excluded. Only cancer types with at least 5 exposed cases with respect to pre-pregnancy diabetes were considered for inclusion in our study. AML was also included since ALL and AML are thought to have distinct etiologies. The final sample included 11,149 cases and 270,147 controls. We examined the childhood cancer types classified according to their respective International Classification of Childhood Cancer, 3rd edition (ICCC-3) codes [39 (link)]: 5,034 leukemias (codes 011–015) of which 4,101 were ALL (code 011) and 706 were AML (code 012), 990 astrocytomas (code 032), 709 intracranial and intraspinal embryonal brain tumors (code 033), 445 germ cell tumors (code 101–105), 337 hepatoblastomas (code 071), 1,378 neuroblastomas (code 041), 741 retinoblastomas (code 050), 463 rhabdomyosarcomas (code 091), and 1,052 Wilms’ tumors (code 061). Our study used de-identified records so we were not required to obtain informed consent. The institutional review boards of University of California Los Angeles and the Committee for the Protection of Human Subjects approved this study.