OBJECTIVES: To examine recent trends in preterm birth (PTB) and its clinical subtypes by maternal race/ethnicity.

METHODS: Kaiser Permanente Southern California electronic health records (EHR) for all births between 2009-2020 (n=433,012) were used to examine PTB trends and its subtypes (spontaneous [SPTB] and medically indicated [MIPTB]). Data on preterm labor triage extracted from EHRs using natural language processing were used to define PTB subtypes. Maternal race/ethnicity was categorized as White, Black, Latina, and Asian/Pacific Islander. Multiple logistic regression was used to quantify the linear trend for PTB and its subtypes. Racial/ethnic trends were further examined by considering statistical interactions and stratifications.

RESULTS: From 2009 to 2020, the overall PTB rate decreased by 15% (from 8.86% to 7.54%; p<.001). Rates decreased by 26% among Whites (from 8.39% to 6.21%, p<.001), 2% among Blacks (from 10.92% to 10.65%; p=.002), 11% among Latinas (from 8.48% to 7.58%; p<.001), and 20% among Asian/Pacific Islanders (from 9.72% to 7.73%; p<.001).

Between 2009 and 2020, overall SPTB rates decreased by 28% (from 6.27% to 4.51%; p<.001). However, overall MIPTB rates increased by 17% (from 2.59% to 3.03%; p=.011). SPTB rates decreased by 35% among Whites (from 6.11% to 3.95%, p<.001), 27% among Blacks (from 7.59% to 5.55%, p<.001), 24% among Latinas (from 5.94% to 4.53%, p<.001), and 30% among Asian/Pacific Islanders (from 6.86% to 4.81%, p<.001). MIPTB rates increased by 20% among Latinas (from 2.54% to 3.06%, p=.006), and 53% among Blacks (from 3.33% to 5.10%, p=.024). Whereas the MIPTB rates for Whites and Asian/Pacific Islanders remained stable at 2.37% and 2.81%, respectively.

CONCLUSIONS: The overall PTB rate decreased over time and was largely driven by a decrease in the SPTB rate. There is racial/ethnic variability in rates of SPTB and MIPTB. The increase in MIPTB among Blacks and Latinas is disconcerting and needs further investigation.