Impact of increased neonatal survival on postneonatal mortality in the United States.

Neonatal intensive care has increased neonatal survival, but has also led to postponement of some of the neonatal deaths to the postneonatal period, particularly in very low birthweight (< 1.5 kg) infants. Our report assesses the impact of the increased neonatal survival and the accompanying delayed deaths on the crude postneonatal mortality rate of the US, using the national livebirth cohort data of 1960, 1980, and 1986. With increased neonatal survival, very low birthweight infants comprised 0.68% of all neonatal survivors in 1986, compared with only 0.31% in 1960. However, postneonatal mortality was increased in infants with birthweights < 1.0 kg from 69 per 1000 neonatal survivors in 1960 to 116 per 1000 in 1986. All other birthweight groups (> 1.0 kg) showed significant reductions in their postneonatal mortality, although the 1.0-1.5 kg group showed the least improvement. Thus, in 1986, 12.1% of all postneonatal deaths were from the very low birthweight neonatal survivors, as compared with 2.7% in 1960. If there had been no improvement in neonatal survival of very low birthweight infants since 1960, the crude postneonatal mortality rate of the US would have been 5.5% and 7.9% less than the actual rates of 3.65 and 3.45 per 1000 neonatal survivors in 1980 and 1986, respectively. However, the impact of these delayed deaths in very low birthweight infants was far less than the increase in their neonatal survival: an additional 416 per 1000 very low birthweight infants survived to 1 year of age in 1986 as compared with 1960. Delayed deaths in the 1.5-2.5 kg birthweight group had a very small effect on postneonatal mortality and there was no such effect of delayed deaths in the > 2.5 kg birthweight group.