Parity progression ratios
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 , can be achieved by using the following formula:

In more developed countries where two-child families are seen as the norm  (or the proportion of women with two children who go on to have a third) is of critical importance in determining overall fertility levels. is simply the proportion of women who become mothers. References 

Human pregnancy