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Last Edited: 8/11/2010

Infant and Fetal Mortality

What are infant and fetal mortality?

Compared to other age groups, mortality (death) during fetal and infant periods occurs because of a relatively small number of causes.  The leading causes change, however, depending upon the precise time period that is considered.  In order to understand discussions of infant and fetal mortality, it is important to know how researchers define each time period.

Infancy is the period beginning at birth and spanning up to 12 months of age.  The neonatal period begins at birth and spans the first four weeks (28 days) of life.  During the neonatal period, death is most common among children that were born preterm and/or having low birthweights and arises from complications of these conditions.  The postneonatal period begins at four weeks of age and spans the rest of infancy.  During the postneonatal period, death is most commonly due to Sudden Infant Death Syndrome (SIDS), a poorly understood situation in which other, more obvious causes of death have been excluded.  The second most common cause of death for both the neonatal and postneonatal periods are conditions arising from congenital malformations or chromosomal abnormalities with which the child was born.

The perinatal period begins during pregnancy before the child is born; it begins once 28 weeks of pregnancy have been completed and spans until seven days following birth (note that by this definition, the perinatal and neonatal periods overlap).  There are a few reasons why researchers find it useful to consider this as a distinct period for study.  First, when a fetus or infant dies around the time of labor and delivery, it is not always clear whether to classify this event as a live birth (and therefore a neonatal death) or a fetal death.  Diagnostic ability for detecting signs of life, such as respiratory effort or heartbeat, umbilical cord pulse, or voluntary movement after delivery may vary between obstetrical facilities. 

Most often, perinatal death occurs among children that have not grown sufficiently during pregnancy, although frequently the cause is unexplained.  It is important to note that death during this period is an important contributor to overall mortality, occurring much more often, for example, than SIDS does during the period of infancy.

Uncertainties regarding causes of death

Researchers frequently classify infant and fetal mortality by age at death rather than cause; one of the reasons for this is that causes of death may be difficult to determine or classify.  Adverse conditions at birth, such as prematurity or growth retardation, heavily influence the risk of death during infancy, although these may be technically separate from the acute cause of death. 

The classic example of diagnostic uncertainty is SIDS, which is by definition mortality lacking medical explanation.  Risk of SIDS appears to peak between the ages of four and 16 weeks and decline thereafter.  Rates of documented SIDS cases have been declining in the USA since the “Back to Sleep” campaign in 1991, which was based on the observation that infants sleeping on their backs had a lower risk of SIDS.1

While the “Back to Sleep” campaign appears to have reduced SIDS incidence substantially, some have observed that much of the decline could also be explained by changes in diagnostic practices surrounding SIDS in which a portion of infant deaths may be assigned other categories.2-4  Some researchers have compared SIDS to unexpected deaths for which an explanation is found, such as acute infection; they have noted that many of the risk factors for SIDS, such as prematurity or household poverty, are the same for unexpected deaths for which causes are found.

1.    Malloy M, Freeman D. Birth weight- and gestational age-specific sudden infant death syndrome mortality:  United States, 1991 versus 1995. Pediatrics. 2000;105:1227-1231.

2.    Leach C, Blair P, Fleming P, et al. Epidemiology of SIDS and explained sudden infant deaths. Pediatrics. 1999;104(4):e43.

3.    Malloy M. Sudden infant death syndrome among extremely preterm infants:  United States 1997-1999. Journal of Perinatology. 2004;24:181-187.

4.    Malloy M, MacDorman M. Changes in the classification of sudden unexpected infant deaths:  United States, 1992-2001. Pediatrics. 2005;115(5):1247-1253.