Season of birth in the population of Norway, with particular reference to the September birth maximum.

The monthly number of births in Norway fluctuates between a maximum in January-May and a minimum in October-December. This cyclic seasonality is assumed to be of biological origin, but indirectly it is influenced by social factors and consequently tends to vary a great deal. There is a secondary birth maximum in September, corresponding to a peak of conceptions during the traditional festivities around Christmas and New Year. It is shown that this maximum is much less variable within the country studied, though it probably varies from one country to another in relation to the impact of the mid-winter traditions. The January-May birth maximum is known to be higher in schizophrenia than in the general population, while no such difference is observed in Norway for the September birth peak. It is felt that there is a fundamental difference between the two birth maxima and a corresponding difference between schizophrenic patients and the general population. Minimal paranatal brain damage of seasonal origin is suggested as an explanation.