infant mortality rate the number of infants who die within one year of their birth, per thousand live births. As in most industrial societies, in the UK the infant mortality rate has markedly declined. In 1901 it was over 150 per 1000 live births, in 1940 it was 61 per 1000 live births, in 1950 it was 31.2 per 1000 live births, and in 1988 it was 8.8 per 1000 live births. Infant mortality rates are an important indicator of the level of economic development, welfare provision, etc. within a society, and class-specific infant mortality rates are an important indicator of social inequalities.