Recurrence Risk of Preterm Birth in Successive Pregnancies Based on Its Subtypes.

Preterm birth (PTB) remains one of the biggest public health challenges with both obstetric and perinatal implications. While a prior PTB is a known risk factor for recurrence, the understanding of the influence of factors such as race/ethnicity, gestational age, PTB subtypes, and interpregnancy intervals (IPI) remains limited. This study aimed to assess whether these factors modify PTB recurrence risk.We conducted a retrospective cohort study of singleton pregnancies in Kaiser Permanente Southern California (2009-2022) using electronic health record data from 82,610 women with two pregnancies and 14,925 women with three. PTB subtypes, spontaneous (sPTB) and indicated (iPTB), were identified through natural language processing. Logistic regression was used to estimate adjusted odds ratios (aOR) and 95% confidence intervals (CI).A first PTB was associated with a sixfold increased risk of PTB in the second pregnancy compared with an uncomplicated pregnancy (23.29 vs. 4.98%, respectively; aOR: 5.60; 95% CI: 5.23-5.99). Those with a history of sPTB (aOR: 5.32; 95% CI: 4.87, 5.81) and iPTB (aOR: 8.26; 95% CI: 7.18, 9.50) had increased risk for the same respective subtype at their second pregnancy. PTB recurrence risk persisted across race/ethnicity categories. In women with PTB in both prior pregnancies, the risk for PTB in a third pregnancy was significantly higher (aOR: 14.59; 95% CI: 11.28-18.88). The recurrence of PTB between first and second pregnancy was substantially higher for those who delivered at 20 to 33 weeks of gestation, regardless of PTB subtype. Non-Hispanic Black and Asian/Pacific Islander women had higher recurrence risk compared with non-Hispanic Whites.These findings highlight disparities in PTB recurrence by race/ethnicity and PTB subtype among a large integrated healthcare system in Southern California, underscoring the need for targeted interventions, particularly for sPTB. · PTB recurrence risk is higher for the same subtype (spontaneous or indicated) in successive pregnancies.. · Non-Hispanic Black and Asian/Pacific Islander women have higher PTB recurrence risks than non-Hispanic Whites.. · Subtype-specific analysis is essential for understanding PTB recurrence, especially considering maternal race/ethnicity..