A parity progression ratios (PPR) is a measure commonly used in demography to study fertility. The PPR is simply the proportion of women with a certain number of children who go on to have another child. Calculating the PPR, also known as ${\displaystyle a_{x))$, can be achieved by using the following formula:

${\displaystyle a_{x}={\text{(women with at least ))x+1{\text{ children ever born) ))/{\text{ (women with at least ))x{\text{ children ever born)))}$^[1]

In more developed countries where two-child families are seen as the norm ${\displaystyle a_{2))$ (or the proportion of women with two children who go on to have a third) is of critical importance in determining overall fertility levels.

${\displaystyle a_{0))$ is simply the proportion of women who become mothers.