The changing pattern of infant mortality in the US: The role of prenatal factors and their obstetrical implications

Infant mortality is one of the leading public health problems in the United States today. During the first half of this century the decline in infant mortality resulted largely from a reduction in postneonatal deaths (2–11 months after birth). Since 1950, two-thirds of all infant deaths have occurred in the neonatal period (1st month after birth). Since 1981, the rate of decline in the infant mortality rate has slowed due to a deterioration in the distribution of birthweights and a slowed improvement in birthweight-specific mortality rates. The role of birthweight is central to this issue, because low birthweight (LBW, < 2500 g) is a major determinant of death in the first year of life, particularly during the neonatal period. Stated another way, less than 0.5% of infants with birthweights > 2500 g die during the first year of life compared to 10.2% of infants with birthweights <2500 g and 45.3% with birthweights < 1500 g (very low birthweight, VLBW). These effects are magnified when evaluated on a race-specific basis: the rate of LBW is twice as high and the rate of VLBW is three times as high for black infants compared to white infants. Reducing the rates of VLBW and LBW, particularly among blacks, holds the greatest potential for future reductions in infant mortality in the United States. The important role of maternal factors in the antecedents of infant mortality (VLBW, LBW, intrauterine growth retardation, preterm birth) have been clearly and repeatedly demonstrated. Some of these factors, such as maternal race, adverse obstetrical history and low level of education, are not amenable to change during pregnancy. Other factors, such as cessation of smoking, higher maternal weight gain and the initiation of early prenatal care have been shown to improve the course and outcome of pregnancy and subsequently result in reduced infant mortality.