Under-5 deaths are usually divided into age groups:
Neonatal mortality includes all live born infants who die in the first 28 days of life. Often neonatal deaths are further divided into early neonatal deaths (deaths in the first 7 days of life) and late neonatal deaths (deaths between 8 and 28 days of life). The neonatal mortality rate is, therefore, the number of live born infants who die in the first 28 days of life per 1000 live born infants.
The neonatal mortality rate is the number of infants who are born alive but die in their first 28 days of life per 1000 live born infants.
Infant mortality includes all the children who die before the age of 1 year (i.e. before reaching their first birthday). Therefore the infant mortality rate is the number of children who die under 1 year per 1000 infant born that year.
The infant mortality rate is the number of infants who die before the age of 1 year per 1000 liveborn infants.
Sometimes the infant mortality rate is divided into the neonatal mortality rate and the post neonatal mortality rate (the number of infants who die between 1 month and 12 months per 1000 infants born that year). The child mortality rate is the number of children aged 1 to 4 who die per 1000 live births. The 5–18 mortality rate includes all the children dying from 5 up to but not including 18 years.
All the childhood mortality rates are expressed per 1000 liveborn infants.