Births, Seasonality of

In a comprehensive analysis of Canadian birth records from 1881 to 1989, it was discovered that there had been a shift in the seasonal distribution of births. The earlier pattern (1881) is characterized by a trough of births during the spring, with most births occurring in the months of January to March. The latter pattern of seasonal births from the early 1920s onwards has remained fairly constant, with most births occurring in the spring and early summer, while relatively few take place in late fall and the winter months. A characteristic feature of the "contemporary" pattern is seen throughout all the Canadian provinces.

Many explanations exist for this phenomenon, ranging from the biological (eg, hormone fluctuations over the seasons) to the sociological (eg, preference for modern couples to link the birth of their children in spring and summer). For the most part, the "early" pattern of seasonality likely reflects natural fertility conditions in the absence of conscious use of contraception by couples. In other words, this pattern reflects conditions that would prevail in a predominantly agrarian rural society. The "contemporary" phenomenon is more indicative of a preference for husbands and wives to time childbirth during the most desirable times of the year, which in industrial societies seems to be spring and summer. The widespread availability and use of efficient contraception facilitates this general predisposition among couples.

The rise of births in September is thought to reflect the influence of the Christmas holidays, where because of the festive atmosphere, a significant number of unplanned conceptions occur, resulting in a rise in births 9 months later. This phenomenon has also been observed in other industrial countries as well.