Infant mortality is defined as the number of infants who are born alive, but die before one year of age. The most common cause of infant mortality worldwide has traditionally been dehydration from diarrhea. Because of the success of spreading information about Oral Rehydration Solution (a mixture of salts, sugar, and water) to mothers around the world, the rate of children dying from dehydration has been decreasing and has become the second most common cause in the late 1990s. Currently the most common cause is pneumonia. Major causes of infant mortality in more developed countries include congenital malformation, infection and SIDS.
Infanticide, abuse, abandonment, and neglect may also contribute to infant mortality.
Related statistical categories:
Infant mortality rate (IMR) is the number of newborns dying under a year of age divided by the number of live births during the year. The infant mortality rate is also called the infant death rate. In past times, infant mortality claimed a considerable percentage of children born, but the rates have significantly declined in the West in modern times, mainly due to improvements in basic health care, though high technology medical advances have also helped. Infant mortality rate is commonly included as a part of standard of living evaluations in economics.
The infant mortality rate is reported as number of live newborns dying under a year of age per 1,000 live births, so that IMRs from different countries can be compared. A good source for the most recent IMRs as well as under 5 mortality rates (U5MR) is the UNICEF publication 'The State of the World's Children' available at http://www.unicef.org/publications/index_18108.html. For example, the worst U5MR is 284 in Sierra Leone. (That is, 28% of all children born die before they turn 5 years old.) The 29 countries with the highest U5MRs are in Africa. The U5MR of the United States is 8, and there are 31 countries with lower U5MRs, although many of those use a less stringent definition of mortality than the US. Sweden's is among the lowest at 3.

Perinatal mortality only includes deaths between the foetal viability (28 weeks gestation) and the end of the 7th day after delivery.
Neonatal mortality only includes deaths in the first 28 days of life.
Post-neonatal death only includes deaths after 28 days of life but before one year.
Child mortality includes deaths within the first five years after birth. Infant mortality rate in countries