Associations of Maternal Age- and Parity-Related Factors With Trends in Low-Birthweight Rates: United States, 1980 Through 2000

OBJECTIVES: We assessed the effects of changes in the maternal age-parity distribution and age-and parity-specific low-birthweight rates on low-birthweight trends in the United States. METHODS: We used natality file data from 1980 through 2000 to assess very-low-birthweight and low-birthweight rates among singleton live-born infants. RESULTS: Changes in age-and parity-specific low-birthweight rates were the main contributor to the overall trend in rates. However, changes in the age-parity distribution, primarily delayed childbearing, had a smaller but noticeable impact. The very-low-birthweight rate increased 27% among Black women, and changes in the age-parity distribution were associated with, on average, more than 20% of the increased rate during the 1990s. Among Hispanic and non-Hispanic White women, on average, more than 10% of the rate increase observed during the 1990s was associated with changes in the age-parity distribution. CONCLUSIONS: Assuming minimal changes in age-specific rates, delayed childbearing may play an increasingly important role in low-birthweight trends in the United States.