Towards a unified perinatal theory: Reconciling the births‐based and fetus‐at‐risk models of perinatal mortality

BACKGROUND: There is a need to reconcile the opposing perspectives of the births‐based and fetuses‐at‐risk models of perinatal mortality and to formulate a coherent and unified perinatal theory. METHODS: Information on births in the United States from 2004 to 2015 was used to calculate gestational age‐specific perinatal death rates for low‐ and high‐risk cohorts. Cubic splines were fitted to the fetuses‐at‐risk birth and perinatal death rates, and first and second derivatives were estimated. Births‐based perinatal death rates, and fetuses‐at‐risk birth and perinatal death rates and their derivatives, were examined to identify potential inter‐relationships. RESULTS: The rate of change in the birth rate dictated the pattern of births‐based perinatal death rates in a triphasic manner: increases in the first derivative of the birth rate at early gestation corresponded with exponential declines in perinatal death rates, the peak in the first derivative presaged the nadir in perinatal death rates, and late gestation declines in the first derivative coincided with an upturn in perinatal death rates. Late gestation increases in the first derivative of the fetuses‐at‐risk perinatal death rate matched the upturn in births‐based perinatal death rates. Differences in birth rate acceleration/deceleration among low‐ and high‐risk cohorts resulted in intersecting perinatal mortality curves. CONCLUSION: The first derivative of the birth rate links a cohort's fetuses‐at‐risk perinatal death rate to its births‐based perinatal death rate, and cohort‐specific differences in birth rate acceleration/deceleration are responsible for the intersecting perinatal mortality curves paradox. This mechanistic explanation unifies extant models of perinatal mortality and provides diverse insights.