Fertility, Age Patterns of

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For biological and social reasons, the probability of a woman's having a child in a given time interval is strongly influenced by her age. The age pattern of fertility of a woman's cohort (that is, the group of women born at the same time) represents the sequence of births and inter-birth intervals over that cohort's reproductive life span. However, the data needed to calculate cohort age patterns of fertility may be obtained only from special sample surveys or population registers. Period age-specific fertility rates, which are derived from data that are more widely available, relate the annual number of births to women of a particular age group (usually one-year or five-year age groups) within the reproductive age range (15 to 49) to the mid-year population size of that age group. (Age-specific fertility rates may also be computed for men, although that is infrequently done.) The set of such period rates for a given time necessarily refers to many different cohorts. The sum of those rates is the (period) total fertility rate. The percentage contributions of age-specific fertility rates to the total fertility rate describe the age pattern of fertility. Because the age-specific fertility rates are not affected by the age composition of the population, they are suitable for comparing age patterns of fertility between populations and over time. Monitoring the age patterns of fertility is important for understanding fertility levels and trends.

Fertility Age Patterns

High fertility is associated with childbearing starting early and continuing until the late reproductive years. However, since women in high-fertility populations typically keep bearing children as long as they remain fecund, the age pattern of fertility is relatively flat. Fertility transition occurs through decreases of fertility at both ends of the reproductive life span, resulting in a more convex age pattern. The decrease of fertility at younger ages is often the result of rising ages at marriage, whereas fertility declines at the older ages primarily because of an increasing propensity to limit family size. The balance of these two trends is context-specific and leads either to "aging" or to "rejuvenation" of the age pattern of fertility.

Age Patterns and the Fertility Transition

Different age patterns of fertility are illustrated in Figure 1 for three groups of countries that in the late 1990s were at different stages of the fertility transition. In the pre-transitional least-developed countries (most of them in sub-Saharan Africa) the total fertility rate averages 5.5 children per woman, 40 percent of which is contributed, on average, by women of age 30 and older. For other developing countries that are progressing through the fertility transition, with total fertility rates averaging 2.8 children per woman, this share drops to 30 percent. In the developed countries, where the demographic transition has been concluded and average total fertility rates are 1.6 births per woman, the share has risen again, to 35 percent.

Fertility often decreases more at older ages than at younger ages during the early stages of the fertility transition, thus resulting in a lowering of the mean age at childbearing and rejuvenation of the fertility pattern. This happened in most developed countries from the late nineteenth century until the 1970s. At the early stages of the fertility transition, a similar


pattern of change was typical for many developing countries of south central and Southeast Asia and Latin America and the Caribbean. Declines in fertility at young ages due to increasing age at marriage initially had a smaller impact on total fertility in comparison to the effects of stopping behavior among older women.

At the later stages of the fertility transition, the balance of these two influences typically shifts, resulting in an increase in the average age of childbearing. In northern Africa and several western Asian countries, the greater part of the fertility decline from the high levels of the 1970s or 1980s to the intermediate or low levels of the 1990s was caused primarily by rising age at marriage, which in the absence of extramarital fertility led to later childbearing.

In the developed countries, fertility decline to replacement level was achieved mostly through a decreasing incidence of high-parity births to relatively older women. Faster decreases of fertility at older ages outweighed the effect on the average age of childbearing of the increasing age at first birth. As a result, the age pattern of childbearing rejuvenated. But the trends toward a younger age pattern of fertility reversed in most developed countries since the


1980s. The aging of age patterns of fertility was especially pronounced in the 1990s. Below-replacement age patterns of fertility are characterized by low levels of fertility among young women (women in their twenties) because of postponement of childbearing until the early and even late thirties and by the fact that large proportions of women stop having additional children after one or two. The shift toward such a pattern has caused the average age at childbearing to rise: Data for around the year 2000 show fertility typically peaking in the age interval from 25 to 29 years; in some countries the peak is in the age group from 30 to 34. In Eastern Europe, however, the age at first birth traditionally was and has remained lower than in other developed regions; consequently, the age pattern of fertility there is young, with the highest fertility in the age group 20 to 24, as shown in Figure 2.

Significance of Age Patterns

Aging of childbearing has an important impact on fertility levels and trends. When childbearing starts before age 20, as is typical in developing countries, the period available for childbearing lasts for approximately 24 years and the period of high fecundity (lasting until around age 35) is 17 to 18 years long. Shortening of the childbearing period in women's lives is an important determinant of persistent below-replacement fertility in many developed and an increasing number of developing countries. In fact, although earlier menarche lengthens the fecund life span, postponement of first births until around age 30 shortens the effectively used fecund period to 12 years and the effectively used period of high fecundability to just 5 to 6 years. When young women postpone births and subsequently try to make up these postponed births in their late 30s or beyond, they are often confronted with increasing likelihood of failure to conceive. Thus, shifts in the schedule of childbearing toward older age depress fertility levels.

See also: Adolescent Fertility; Mortality, Age Patterns of; Natural Fertility.


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——. 2000. World Population Prospects: The 2000 Revision. CD-ROM Sales No. E.01.XII.3. New York: United Nations.

Serguey Ivanov

Vasantha Kandiah