The fertility rate is calculated from the number of live births of children over the female population aged between 15 and 49. Demographers have been able to estimate what a natural fertility rate could be. Various factors may affect the fertility rate, even in the absence of contraception or abortion, and explain the wide fluctuations of the rate that may be found over time or place. We only address the most important factors here, as they especially impacted on Quebec. A first factor that served to reduce the fertility rate, and hence the birth rate , is age.
Greater fertility is associated with a younger age.
Beyond a certain age, which may vary from individual to individual, fertility declines rapidly until a point when the woman would become sterile. For some reason, a territory, or a given population, may have an older population, and hence lower fertility and birth rates. Areas of emigration are frequently in such a position as the young are more likely to be mobile, leaving behind and older population. To illustrate the point in the absurd, one would assume the fertility rate to be nil in an old folks' home, or in a retirement community. In a traditional Catholic society, such as Quebec was for a long time, sex was only permissible within marriage.