Which birthweight groups contributed most to the overall reduction in the neonatal mortality rate in the United States from 1960 to 1986?

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Abstract
We determined the relative contributions of individual birthweight groups to the reduction in neonatal mortality rate (NMR) of US singleton livebirths from 1960 to 1986, the period during which neonatal intensive care was introduced and became established. Changes in the NMR for each race (non-white/white) and birthweight group were assessed in terms of three components of NMR (1) birthweight-specific neonatal mortality rate (BWS-NMR), (2) birthweight distribution, and (3) the interaction resulting from simultaneous changes in BWS-NMR and birthweight distribution. Overall, NMR decreased from 16.7 per 1000 livebirths in 1960 to 5.5 per 1000 livebirths in 1986. This reduction was achieved mainly by improvements in the BWS-NMRs of all birthweight groups. Neonates who weighted > 1.5 kg at birth were the major contributors to the overall reduction in the NMR; approximately two-thirds of total reduction in NMR between 1960 and 1980 and 52.6% of the total reduction between 1980 and 1986 occurred in the > 1.5 kg birthweight groups. Our study demonstrates that with the introduction of neonatal intensive care, survival of infants with birthweights > 1.5 kg improved markedly and that this improvement accounted for most of the decline in the neonatal mortality rate in the United States from 1960 to 1986. Therefore, in measuring the effectiveness of neonatal intensive care, the assessment should include not only very low birthweight (< 1.5 kg) but all birthweight groups.