Population projections require assumptions about future age-specific fertility rates, which are derived from assumed TFRs and age distributions of fertility. These rates are applied to the projected female population in each year of the projection period in order to determine future numbers of births, and therefore the size of future projected populations.
Over the past 10 years, age-specific fertility rates have been declining for the younger age groups (women below age 30), whilst increasing among women aged 30 years and over, representing a gradual shift in fertility towards older ages.
The projected age distribution of mothers is based on half the average rate of change in the age-specific fertility rates during the period 2012–2016. The historical rate of change is assumed to slow down due to limits on child-bearing ages. These trends are assumed to continue under all three fertility scenarios until 2027, after which the age pattern of fertility remains constant.
Linear interpolation is used to obtain TFRs for each year 2017 to 2026 for all three scenarios, using the known TFR for 2016 and assumed TFR for 2027. To create assumed age-specific fertility rates, the assumed distribution of age of mothers is then applied to the assumed TFR for the corresponding projection year.