Attention will be focused in this chapter on the reproductive history of certain groups of women as they live through the childbearing period. These groups will be compared with respect to their lifetime fertility and also with respect to their fertility up to and during younger ages. Two of the measures of fertility which will be used are very similar to two which have been used in preceding chapters, namely, cumulative birth rates up to various ages and distributions of women by the number of children they have borne. Here, however, we shall not be restricted to rates and distributions based on data collected in decennial censuses or in the Census Bureau's Current Population Surveys, which are available only for 1910, 1940, 1950, 1952, and 1954. Instead, we shall consider those derived from annual numbers of births for each year since 1910 and related census data.
The groups of women to be considered are those born in successive years, and will be referred to as birth cohorts. For example, the girls born during the twelve months centering on January 1, 1900, make up the birth cohort of 1900, those born during the twelve months centering on January 1, 1901, make up the birth cohort of 1901, etc. Most of these girl babies grew up and had one or more babies themselves. The consideration of the various relationships between the number of the original girl babies (or their survivors at a given age) and the number of babies they bear is the essence of cohort fertility analysis. For some countries it is possible to classify women by the year in which they marry and to study the fertility of marriage cohorts. For the United States, however, the information needed to measure and analyze fertility year by year is available for birth cohorts but not for marriage cohorts. From the standpoint of collecting data on the number of marriages by age, race, and marital status of bride and on the number of births by duration of marriage, the United States has been a backward country. Since only birth cohorts can be considered in this chapter, the term "birth cohort' will be shortened to "cohort" in most places.
The chief advantage of fertility rates for real cohorts is that they show changes in the number of children actually borne per woman--informa-