The postponement of neonatal deaths into the postneonatal period: evidence from Massachusetts.

Part of the slow decline in the postneonatal mortality rate and the rapid decline in the neonatal mortality rate during the 1970s may have been due to a postponement of some neonatal deaths into the postneonatal period. The authors hypothesized that any such postponement should be accompanied by a lack of decline, or even an increase, in late neonatal and postneonatal mortality rates among low birth weight babies and babies dying of conditions originating in the perinatal period. To examine this theory, the authors used vital records data to compare infant mortality rates in Massachusetts during 1970-1972 with rates during 1978-1980. Log-linear hazard models were used to calculate death rates, while controlling for changes in maternal age, race, education, and prior reproductive history. The authors found that babies of birth weight under 1,500 g had no decline in late neonatal mortality rates and babies of birth weight under 2,500 g had no decline in postneonatal mortality rates. Babies of birth weight 500-999 g had an increased postneonatal mortality rate (rate ratio = 2.4; 95% confidence limits = 1.0-5.4). These unimproved or increased death rates were due in part to conditions originating in the perinatal period. The authors conclude that, although infant mortality rates have declined, this postponement was real, and that efforts to monitor infant mortality will benefit from its routine quantification.