Age at last birth and its components
This paper examines the ways in which the behavior of twentieth century cohorts of American women changed simultaneously in the three components of fertility that determine age at last birth–age at first birth, spacing between subsequent births, and parity progression ratios of subsequent births–to produce changes in the timing of the completion of childbearing. It decomposes changes in the mean age at last birth among cohorts and between whites and nonwhites to changes in these three components. They use fertility rates available from vital statistics data. They covered the cohorts of 1903 to 1940 and they projected each age component of age at last birth for these cohorts, under the assumption that these cohorts would experience the fertility rates of the calendar year 1986. Results show that only ages at first birth consistently contributed to changes in age at last birth in the same direction. In general, the cohorts increased and decreased their age at first birth, birth intervals, and parity progression ratios of lower and higher birth orders in almost every possible combination so as to achieve a relatively young age at final birth. It is interesting to note that the mean age at last birth was 30.9 for the 1903 cohort, rose to 32 for the 1917 cohort and decreased rapidly to 28.2 projected to the 1943 cohort. These are relatively young mean ages. Just to give an idea the mean age at last birth in developing countries in the 1970's and 1980s were estimated in 39.5 for African countries, 36.9 in Latin America and 39 for the Middle East.