Infant mortality refers to death within the first year of life to persons born alive. A live birth is defined as the "complete expulsion or extraction from its mother of a product of
4 For a comprehensive discussion of the social origins and of the analytic importance of race and ethnicity as key dimensions in social science theory, see Bonilla-Silva (1999).
conception, irrespective of the duration of pregnancy, which after such separation breathes or shows any other evidence of life'' and death as "the permanent disappearance of life any time after live birth has taken place'' (United Nations 2001: 13). The infant mortality rate is conventionally measured as the number of deaths to infants under one year of age in a given year, per 1,000 live births in the same year. However, not all infant deaths occur in the calendar year of birth, so that the IMR as just defined is not a probability based on the population at risk. A true probability can be computed directly from birth cohort data of the sort routinely made available in the Linked Birth/ Infant Death Files by the US National Center for Vital Statistics (NCHS). Shryock and Siegel (1976) refer to the computation based on birth cohorts as the infant death rate. In the absence of substantial year-to-year fluctuations, the infant mortality rate and the infant death rate will be similar in magnitude. It is possible to apply separation factors that apportion vital events in a manner that moves calculations based on period data closer to what would be achieved with cohort data. Another frequently adopted alternative is the use of multiyear averages that "may serve adequately as adjusted measures'' (Shryock and Siegel 1976: 237). Inasmuch as infant mortality is a rare event in many areas of the world, combining infant deaths occurring over a three-, five- or other multiple-year period and dividing by the corresponding grouped number of live births has often been employed as a way of ensuring that the number of cases is sufficiently large to yield stable estimates, as well as a means of adjustment. Throughout this chapter, the term infant mortality rate (IMR) is employed regardless of whether computations are based on period or cohort data.
Infant mortality is often subdivided into neonatal mortality (deaths to infants under 28 days) and postneonatal mortality (deaths during the remainder of the first year). By the last decade of the 20th century, about 65% of all infant deaths in the U.S. occurred during the first month of life, with the majority of the latter (about 80%) occurring during the first week (National Center for Health Statistics 1996: Table 23). While there is reason to be interested in the timing of infant death, per se, researchers have also used this dichotomy to proxy cause of death structure. Specifically, neonatal mortality has been used to approximate deaths due to endogenous causes, i.e., conditions that are related to genetic makeup or that are "a consequence of circumstances occurring during the prenatal period and/or the birth process'' (Frisbie et al. 1992: 535). Exogenous infant mortality is due to environmental or external causes, such as infections, accidents, etc. (Bogue 1969). Although the absence of information on cause of death may force reliance on timing of death, there is evidence that, while the timing proxy may have been of acceptable validity in early research, such is no longer the case (Frisbie, Forbes, and Rogers 1992; Poston and Rogers 1985). For example, although for quite some time exogenous mortality represented more than half of all postneonatal deaths in the U.S., such is no longer the case. The reason that endogenous conditions are now the most prevalent cause of death in the postneonatal period ''is that advances in perinatal care and extraordinary medical intervention... have resulted in the survival of nonviable infants past the first 27 days of life'' (Frisbie, Forbes, and Rogers 1992: 544). In the more distant past, or in areas of the world today where modern perinatal care and technology are not widely available, the timing proxy may (unfortunately) retain a fair degree of validity. Finally, in the absence of cause-of-death data, the timing proxy may permit adequate relative comparisons if temporal trends for two or more groups are homogeneous as to direction of change (Frisbie, Forbes, and Rogers 1992).
Perinatal mortality combines late fetal deaths (often at 28 weeks or longer gestation) with deaths to infants less than one week of age.5 The perinatal mortality rate denotes late fetal and early infant death divided by the population at risk. In other words, the denominator for the rate is the sum of live births plus fetal deaths. Use of only live births in the denominator yields the perinatal mortality ratio (Shryock and Siegel 1976: 246).
Fetal mortality, i.e., death prior to the complete expulsion or extraction of the fetus, is excluded under the definition of infant mortality. Fetal death generally refers to both stillbirth and abortion, whether spontaneous or induced (Shryock and Siegel 1976). Although fetal death as an outcome is not considered further here (because infant mortality can occur only in the case of a live-born infant), previous pregnancy loss has been shown to be an important risk factor for infant mortality.
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