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:

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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.