The longterm historical decline in infant mortality has been accompanied by increasing concentration of infant deaths at the earliest stages of infancy. The influence of prenatal and neonatal conditions has become increasingly dominant relative to postnatal conditions as external causes of death such as infectious disease have been diminished. In the mid-1960s Coale and Demeny developed formulas describing the dependency of the average age of death in infancy on the level of infant mortality from data obtained up to that time.

Almost at the same time as Coale and Demeny’s analysis, as shown in this paper, in the more developed countries a steady rise in average age of infant death began. This paper demonstrates this phenomenon with several different data sources, including the linked individual birth and infant death datasets available from the US National Center for Health Statistics and the Human Mortality Database. A possible explanation for the increase in average age of death in infancy is proposed, and modifications of the Coale-Demeny formulas for practical application to contemporary low levels of mortality are offered.