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The mortality of live-born children who have not yet reached their first birthday is called infant mortality 1. The mortality of live-born children who die before reaching a certain age, taken as four weeks or a month, is called neo-natal mortality 2. It has been suggested that the deaths of children dying after the neo-natal period 3, but before reaching the age of one year might be called post-neo-natal mortality 4. The expression foetal mortality 5 as recommended for instructional use by the World Health Organization is used for deaths prior to the complete expulsion or extraction from its mother of a product of conception (602-6) irrespective of the duration of pregnancy (603-3). The terms mortality in utero 5 or intra-uterine mortality 5 may also be used. Foetal mortality includes miscarriages, abortions (cf. 603-5) and stillbirths 6. Since the causes of death of foetuses (602-7) during the last few weeks of pregnancy and of live-born infants during the first few days of life often result from similar underlying factors, the term peri-natal mortality 7 has been suggested to include stillbirths and neo-natal deaths.
- 3. In English-speaking countries the neo-natal period is taken as 28 days. The term early infancy is occasionally used as an approximate equivalent to neo-natal period, as, e. g., in "diseases of early infancy".
- 5. foetal — American spelling fetal.
- 6. Miscarriages, abortions and stillbirths are collectively referred to as "foetal deaths".
The infant mortality rate 1 is generally computed as the ratio of infant deaths 2 (i. e., the deaths of children under one year of age) registered in a given year to the total number of live births registered in the same year. This rate is only an approximate measure of the true risk of death between birth and first birthday. If infant deaths are classified by year of birth and by year of death it is possible to obtain a better approximation to that risk. In the absence of such information separation factors 3 may be estimated, which divide infant deaths into those occurring to infants born in the current calendar year and to infants born in the previous calendar year. Stillbirth rates 4 and foetal death rates 5 are computed in the same way as infant mortality rates, except that all births are used in the denominator and not live births only.
- 5. A foetal death ratio is sometimes computed, showing the ratio of the number of foetal deaths to the number of live births.
In the study of age-specific mortality 1 the terms infant mortality (410-1) and neo-natal mortality (410-2) are the only terms that refer to generally accepted age periods. The usage of such terms as child mortality 2, adult mortality 3 or mortality of old age 4 is not uniform.
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