Aim: To study the seasonality of month of birth among African American children with insulin-treated diabetes mellitus (DM) in the city of Chicago, in order to determine whether perinatal exposures play a significant role in diabetes risk among children of non-European origin.
Method: The Chicago Childhood Diabetes Registry ascertains new cases of insulin-treated DM among minority children < 18 years of age; these cases were compared with birth certificate data for the general African American population in Chicago. The chi2 test and Poisson regression were used to compare the pattern of month of birth of children with DM (n = 604) to that of the general population (n = 758,658) over the same period of years (1968-1995).
Results: In a month-by-month comparison, there were significantly fewer children who later developed DM born during October (chi2 = 6.74, df = 1). This seasonal pattern was stronger among males (n = 284) than females (n = 320), and among those who apparently developed type 2 DM (n = 155) compared to those who developed type 1 DM (n = 449). Children who were diagnosed between 15 and 17 years of age (n = 131) demonstrated significant seasonality (chi2 = 27.6, df = 11) compared to the general population.
Conclusions: The apparent protective effect of October birth, and the significant overall seasonality among those diagnosed at ages 15-17 years, suggest the possibility that seasonal environmental factors at conception, during pregnancy or in the neonatal period may affect DM risk in adolescence. The greater impact of month of birth in adolescent type 2 DM patients is surprising and seems to indicate a role for mechanisms other than the immunological ones previously suggested.