Keiser AM, et al. - Among primiparae non-Hispanic blacks (NHB) and non-Hispanic whites (NHW), researchers ascertained if the connection between maternal age and preterm birth (PTB) differs by race/ethnicity. This cross-sectional study was conducted in California from 2008 to 2012. The study sample consisted of 367,081 singleton liveborn first births to NHB and NHW women. For comparisons, universal age/race reference groups (NHW, 25-29 years) and race-specific reference groups (NHW or NHB, 25-29 years) were used. At all maternal ages, primiparae NHBs showed an increased risk of PTB, spontaneous PTB, and clinician-initiated PTB vs NHWs. Rate ratios, however, using race-specific reference groups converged across maternal ages, indicating a comparable independent race/ethnicity impact of maternal age on PTB. A maternal age differential impact does not seem to explain race/ethnicity disparities in PTB.