Cross-Sectional Average Length of Life by Parity: Illustration of US Cohorts of Reproductive Age in 2015

Traditionally, researchers measure fertility changes by analyzing the total fertility rate, mean age at childbirth, and parity progression ratios using period and cohort perspectives. However, the period and cohort indexes have well-known limitations: period indexes use a synthetic-cohort approach, while cohort indexes provide an outdated picture of current fertility patterns because they are based on information on populations that are no longer of childbearing ages. The Cross-sectional Average Length of Life by Parity (CALP) is introduced here as an alternative way of understanding fertility trends. The CALP shows the length of time women spend in each parity during reproductive age and is a period measure including all the cohort fertility information of reproductive-age women at a given time. By selecting the US data from the Human Fertility Database for illustration, the CALP is calculated using a hierarchical multistate life table model. The CALP for the year 2015 shows that women in the US spend 47% (17.91/38 years) of their reproductive years from ages 12 to 50 in childlessness, followed by 16%, 19% and 11% in parities 1, 2, and 3, respectively.