Maternal age and parity-associated risks of preterm birth: differences by race/ethnicity.

There is a well-known interaction between maternal age and parity in the risk of adverse perinatal outcomes, including preterm birth (PTB), such that young multiparae and older primiparae have greater risks. Yet it is not known whether this interaction varies by race/ethnicity. US birth records for singleton births from 2000 to 2002 were used to examine the incidence of PTB by maternal age and parity within non-Hispanic White, non-Hispanic Black and Hispanic subgroups. PTB was categorised as moderately (32-36 weeks), very (28-31 weeks), or extremely (<28 weeks) preterm. Odds ratios of PTB according to age and parity were calculated in racial/ethnic specific multinomial logistic regression models. Within each race/ethnicity, comparisons were made relative to 25- to 29-year-old primiparae. Young teenagers (<18), particularly multiparae, generally had a higher risk of each degree of PTB among all three racial/ethnic groups. However, Black teenagers did not have a higher risk of extremely PTB. For very and extremely PTB, teenagers had considerably higher risk among Whites than Blacks or Hispanics. Within each racial/ethnic group, older (35+ years) primiparae had similarly higher risk of each category of PTB relative to 25- to 29-year-old primiparae. Older multiparae had higher risk of moderately and very PTB among Black and Hispanic women only. Adjustment for education did not alter these findings. Teenagers and older primiparae are already widely regarded as having greater perinatal risks. This study suggests that, among Black and Hispanic women, older multiparae may also have a higher risk of moderately and very PTB.