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ESTIMATES OF FERTILITY are among the most widely used demographic statistics. In many developing countries recent levels and trends in fertility are avidly watched by policymakers, family planning program managers, and demographers to determine whether and how rapidly fertility is moving in the desired downward direction. In much of the developed world, where fertility is now at historic lows, these same statistics are examined for signs of an upturn in fertility back to the replacement level needed to prevent future declines in population size. Given this interest in measuring human reproduction, it is desirable for users of fertility statistics to understand the strengths and weaknesses of available indicators. It is particularly important to avoid basing policies on statistics that give potentially misleading information.Although the demographic literature offers many measures of fertility, the total fertility rate (TFR) is now used more often than any other indicator. The TFR is defined as the average number of births a woman would have if she were to live through her reproductive years (ages 15-49) and bear children at each age at the rates observed in a particular year or period. It is a hypothetical measure because no real group of women has experienced or will necessarily experience these particular rates. The actual childbearing of cohorts of women is given by the completed fertility rate (CFR), which measures the average number of births 50-year-old women had during their past reproductive years. The CFR measures the true reproductive experience of a group of women, but it has the disadvantage of representing past experience: women currently aged 50 did most of their childbearing two to three decades ago when they were in their 20s and 30s. The advantage of the TFR is that it measures current fertility and therefore gives up-to-date information on levels and trends in fertility. Another reason for the popularity of the TFR is its ease of interpretation compared with some other measures. Most interested persons will have littledifficulty interpreting fertility measures expressed in births per woman, but few non-demographers will know intuitively whether populations with a crude birth rate of 10 (births per 1000 population) or a general fertility rate of 100 (births per 1000 women of reproductive age) have high or low fertility.The simplicity and wide availability of the TFR have contributed to a neglect of some deficiencies in this fertility indicator. The demographic literature on the measurement of fertility includes many criticisms of and alternatives to the conventional TFR, but there is no agreement on a replacement for it. In the next section we provide a brief selective review of the past half-century of demographic literature on the subject. This is followed by a proposal for arriving at an adjusted version of the TFR that is free of so-called tempo effects-distortions due to changes in the timing of births. The conventional TFR can be considered to consist of a quantum and a tempo component. We...

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The world and most regions and countries are experiencing unprecedentedly rapid demographic change. The most obvious example of this change is the huge expansion of human numbers: four billion have been added since 1950. Projections for the next half century expect a highly divergent world, with stagnation or potential decline in parts of the developed world and continued rapid growth in the least developed regions. Other demographic processes are also undergoing extraordinary change: women's fertility has dropped rapidly and life expectancy has risen to new highs. Past trends in fertility and mortality have led to very young populations in high fertility countries in the developing world and to increasingly older populations in the developed world. Contemporary societies are now at very different stages of their demographic transitions. This paper summarizes key trends in population size, fertility and mortality, and age structures during these transitions. The focus is on the century from 1950 to 2050, which covers the period of most rapid global demographic transformation.

Conventional theories have little to say about the level at which fertility will stabilize at the end of the demographic transition, although it is often assumed that replacement fertility of about 2.1 births per woman will prevail in the long run. However, fertility has dropped below the replacement level in virtually every population that has moved through the transition. If future fertility remains at these low levels, populations will decline in size and will age rapidly.This paper examines the causes of discrepancies between reproductive preferences and observed fertility. Examples of such deviations are found in many contemporary developed countries, where desired family size is typically two children while fertility is well below replacement. Six factors are identified as the causes of these discrepancies. Of these factors, the fertility-depressing impact of the rising age at childbearing is one of the most important. This factor reduces fertility only as long as the age at childbearing keeps rising. Once the mean age stops rising-as it eventually must-fertility will rise closer to the desired level of two children, because the depressing effect is then removed. The current low levels of fertility in many developed countries may therefore not be permanent.This material may not be reproduced in any form without written permission from the author.