摘要：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 His-panic 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. Rates of low birthweight (LBW) in the United States increased from 1980 through 2000. 1 , 2 During the same period, percentages of births among teenage mothers declined, whereas percentages among mothers 35 years or older increased. 2 – 5 Because there is a U-shaped relationship between maternal age and LBW, 6 – 10 the decreasing percentages observed among teenage mothers should have lowered crude LBW rates over the period, and the increasing percentages among older mothers should have led to a rise in these rates. A number of studies 4 , 6 , 8 , 9 , 11 – 19 have examined changes in maternal age, parity status, and LBW separately; in the present investigation, we assessed changes in these factors simultaneously. The relationships between maternal age, parity status, and LBW are important. If age- and parity-specific LBW rates are constant over time, changes in secular LBW trends may predominantly reflect changes in maternal age and parity, and there may be little intervention potential apart from preventing teenage pregnancies. However, if age- and parity-specific LBW rates change over time, this may reflect shifts in medical practice, environmental exposures, socioeconomic status, or personal lifestyles. We assessed these 2 possible sources of change separately because age- and parity-specific rates are the primary target of public health interventions (e.g., prenatal care clinics) and can be used to assess racial disparities.
