Competing effects on the average age of infant death

In recent decades, the relationship between the average length of life for those who die in the first year of life — the lifetable quantity 1𝑎0 — and the level of infant mortality, on which its calculation is often based, has broken down. The very low levels of infant mortality in the developed world correspond to a range of 1𝑎0 quantities. We illustrate the competing effect of falling mortality and reduction in preterm births on 1𝑎0, through two populations with very different levels of premature birth — infants born to non-Hispanic white mothers and to non- Hispanic black mothers in the United States. Through simulation, we further demonstrate that falling mortality reduces 1𝑎0, while a reduction in premature births increases it. We use these observations to motivate the formulation of a new approximation formula for 1𝑎0 in low- mortality contexts, which is a function of both the infant mortality rate and the ratio of infant to under-five mortality. Model results and validation show that this model outperforms existing alternatives.