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.