Rates of preterm delivery among Black women and White women in the United States over two decades: an age-period-cohort analysis.

The authors assessed the influence of age, period, and cohort effects on rates of preterm delivery in the United States. Rates of preterm delivery for singleton births (<37 weeks) in seven age groups (15-19, 20-24,., 45-49 years), five periods (1975, 1980, 1985, 1990, 1995), and 11 maternal birth cohorts (1926-1930, 1931-1935,., 1976-1980) were examined. Over the 20-year study interval, preterm delivery increased by 3.6% among Blacks (from 15.5% in 1975 to 16.0% in 1995) and by 22.3% among Whites (from 6.9% to 8.4%). Among Black primigravid women, rates of preterm delivery increased from 1975 to 1990 and began to decline thereafter; among Whites, the rates increased between 1975 and 1995. In Blacks, women aged 25-29 years had the lowest rates for the first and second births, and women aged 30-34 years had the lowest rate for subsequent births. In Whites, the age groups with the lowest preterm delivery rates were 20-24 years for first births and 25-29 years for subsequent births. Cohort-specific rates of preterm delivery remained fairly constant across age strata and periods for Whites, but a small trend was apparent for Blacks aged 30-44 years. The consistency of the observed age effects across periods and cohorts suggests that the age effect is partly due to biologic factors. The presence of period effects might be linked to the increased survival of premature infants or to increased viability among births occurring at lower lengths of gestation.