The changing pattern of low birth weight in the United States--1970 to 1980.

Analysis of US Natality Statistics using a bivariate (birth weight-gestational age) approach shows that trends among term-low-birth-weight infants (less than or equal to 2,500 g; greater than or equal to 37 weeks, term low birth weight) and preterm-low-birth-weight infants (less than or equal to 2,500 g; less than 37 weeks, preterm low birth weight) have different patterns over time and by race. Between 1970 and 1980 the incidence of preterm low birth weight for all races declined 7.1%, while the term-low-birth-weight incidence declined almost three times as much (20.9%). The incidence of preterm low birth weight among white infants, during the same period, declined 9.0% with the black preterm-low-birth-weight rate declining by 5.8%. The term-low-birth-weight incidence, however, was 24.6% lower among whites and 14.9% lower among black term-low-birth-weight infants for the same period. The reduction in the overall low-birth-weight incidence for both populations is principally caused by reduction in the incidence of term-low-birth-weight infants. Term- and preterm-low-birth-weight infants reflect different etiologic insults and require different intervention strategies for prevention.