The term "Demographic Transition" describes the movement of human populations into a roughly steady state. Initially birth rates are high, but so are infant mortality and other death rates, and population may rise, but it does so quite gradually. In Europe and North America, the Demographic Transition occurred over two centuries - gradually, as hygenic practices changed, medicine improved and other factors lowered death rates, women grew up noticing "hey,five kids aren't necessary - I can have three and be assured of getting them to adulthood." Thus, the average TFR (that's the total fertility rate) dropped steadily from 6 to 2.8 and then down further. Now the developed world has an average TFR of 1.8, below replacement rate.