Fertility is the process of producing live births. Because of their roles in shaping population dynamics and change, fertility, migration, and mortality are central concepts in the field of demography. Demographic transition theory attempts to trace populations as they move from high or low birth and high or low death rates; however, researchers now recognize that the process is more complicated than the theory suggests.
While counting births can be straightforward, exposure to the risk of becoming pregnant is not universal, complicating the denominator in measures of fertility. Although both men (or at least sperm) and women are necessary to create a live birth, measurement is simplified by relating fertility to one sex, usually women. The crude birth rate, a basic measure, counts the number of live births per 1,000 women of childbearing age (usually 15–49 years old) in the population of interest. The age-standardized fertility rate and total fertility rate (TFR) are two measures that directly account for age variation in fertility. The proximate determinants framework attempts to quantify a woman’s exposure to the risk of becoming pregnant as it varies by four factors: marital/union status (exposure to sexual intercourse), postpartum infecundability (the inability to become pregnant directly after giving birth, as breast-feeding further slows the return of menstruation), contraception use, and abortion (spontaneous and induced).
Fertility levels can be characterized in a number of ways. Populations with high birth rates are often referred to as those with natural fertility, suggesting that there is no conscious effort to control family size. Theoretically, women could begin childbearing in their mid-teens, having a child each year until their late 40s, resulting in 35 births. Yet, even in natural fertility populations like the Hutterites (United States/Canada) and the !Kung (Kalahari Desert), TFR was far below this biological maximum, at 10 to 12 and 4.5 births, respectively. Replacement fertility occurs when TFR is 2.1 (a woman has two children over her lifetime, thus replacing the children’s parents; the one tenth accounts for child mortality under age 15). Today a number of countries, including many in Europe and East Asia, have total fertility rates below replacement: On average women have fewer than two children over their lifetime.
Fertility research often focuses on (a) ways populations control family size, (b) access to and unmet need for modern contraception, and (c) understanding changing fertility levels. An interest in fertility control through natural and modern contraception and the processes behind fertility decision making drives a number of quantitative and qualitative studies in diverse populations. For example, the World Fertility Surveys (60+ countries, 1974–86) and the Demographic and Health Surveys (mid-1980s–present, 70+ countries, many with multiple surveys) provide data to explore some of these issues. Understanding the influence of gender dynamics and social contexts on fertility and the impact of emerging reproductive technologies are central to future understandings of fertility and infertility.
Abortion; Birth Rate; Contraception; Demographic Transition Theory; Population Growth; Total Fertility Rate
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