Distribution of birth dates in type 1 diabetic patients versus nondiabetic controls

The aim of our study was to analyze the temporal relationship between birth date and the onset of insulin-dependent diabetes mellitus (IDDM). Because IDDM is a disease with a strong genetic base, we analyzed in this study the monthly distribution of births of 900 type 1 diabetic patients with onset of diabetes before the age of 20, and 7,839 births registered in 3 consecutive years (1990-1992) in a district of Bucharest, with a population of about 250,000 inhabitants. We observed that the monthly distribution of births in both nondiabetic and diabetic populations was not uniform. In nondiabetic population the maximum number of births was recorded in June (9.57% of all cases) and the minimum one in November (6.45%), giving a difference of percentage between them of 3.12%. In diabetic population the difference was significantly higher than in nondiabetic individuals (Chi square = 3.408, for a confidence of 90%, p < 0.01). The maximum percentage of births was recorded in April (11.56%) and the minimum in January (6.22%) with a difference between them of 5.3%. For diabetic population, there were two months (April and March) in which the number of births was higher (a susceptibility to type 1 diabetes?) and two (January and October) in which the number of births in diabetic subjects was lower (resistance to type 1 diabetes?). If this pattern of seasonality of births in diabetic patients vs nondiabetic individuals will be confirmed on a larger number of subjects, then one possible explanation could be the genetic susceptibility or resistance to type 1 diabetes in different groups of subjects.