The estimated total fertility rate (TFR) is probably smaller than the actual one and the difference between the two has narrowed over the period. Three major components affecting population growth are fertility, mortality and migration, and among these components, fertility plays the most important role. A number of factors, such as social, cultural, economic, health and other environmental factors, directly determine fertility. Davis and Blake (1956) first introduced the term intermediate variables of fertility to describe the biological and behavioral mechanisms through which social, economic and cultural conditions can affect fertility.