In recent decades, the relationship between the average length of life for those who die in the first year of life-the life table quantity a10-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 a10 quantities. We illustrate the competing effect of falling mortality and reduction in preterm births on a10 through two populations with very different levels of premature birth-infants born to non-Hispanic White mothers and infants born to non-Hispanic Black mothers in the United States-using linked birth and infant death cohort data. Through simulation, we further demonstrate that falling mortality reduces a10, while a reduction in premature births increases it. We use these observations to motivate the formulation of a new approximation formula for a10 in low-mortality contexts, which aims to incorporate differences in preterm birth through a proxy measure-the ratio of infant to under-five mortality. Models are built and tested using data from the Human Mortality Database. Model results and validation show that the newly proposed model outperforms existing alternatives.