The growth in train passengers in the late nineteenth century or the rate of growth in the airline industry of the twentieth century are perhaps the closest parallels we have for estimating this growth. This level is used to allow unlimited growth below this level. When the transport needs reach this level then the Transport Growth Rate will constrain future growth. The fertility rate is the average number of babies that a woman will have during her lifetime. Birth rates are commonly measured as the number of births per thousand people in a year.
Birth rates depend on both the fertility rate as well as the age distribution of a population.
For example, two populations with the same fertility rate may have different birth rates depending on the proportion of people between the ages 20-40. In case you're wondering, the death rate declines as the birth rate increases because the proportion of young people is larger so the number of deaths as a percentage of the whole population declines even though life expectancy remains the same. Let's look back during similar periods of expansion. Translating these numbers gives a fertility rate ranging from 2.5 to 4. One other complication in the fertility rate is how its distributed over the lifetime of a woman.