Background: Preterm birth remains one of the biggest public health challenges with both obstetric and perinatal implications

Objective: Although a prior history of preterm birth (PTB) is a potential risk factor for recurrence in successive pregnancies, little information is available on whether recurrence risk is modified by race/ethnicity, gestational age at birth, PTB subtypes, and interpregnancy intervals (IPI); therefore, we examined whether PTB recurrence risk is modified by these factors.

Methods: A retrospective cohort study of singleton pregnancies in Kaiser Permanente Southern California (2009-2022) was conducted using data extracted from electronic health records (EHR) on the first 2 (n= 82,610) and 3 (n=14,925) pregnancies. Data on preterm labor triage extracted from EHRs by implementing natural language processing were used to define PTB subtypes (spontaneous PTB sPTB and iatrogenic PTB iPTB). Logistic regression models were used to estimate adjusted odds ratios (aOR) and their 95% confidence Intervals (CI).

Results: A first pregnancy complicated by PTB was associated with a 6-fold increased risk of PTB in the second pregnancy compared with a first uncomplicated pregnancy (23.29% vs. 4.98%, respectively; aOR, 5.60, 95% CI: 5.23-5.99). Stratified by their subtypes, 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 and PTB subtypes with the highest risks observed for iPTB. Compared to pregnancies without PTB in the first two pregnancies (4.58%), those with PTB in both pregnancies (40.94%) were associated with significantly increased PTB risk in their third pregnancy (aOR: 14.59, 95% CI: 11.28-18.88). The recurrence of PTB between 1st and 2nd pregnancy was substantially higher for those who delivered at 20-33 weeks of gestation, regardless of PTB subtype. The risks of both sPTB and iPTB recurrence in successive pregnancies were unrelated to the length of IPI. Underweight pregnant patients whose first pregnancy was complicated by sPTB and iPTB had higher odds of PTB subtype-specific recurrence in a subsequent pregnancy. Non-Hispanic Blacks and Asian/Pacific Islanders had a higher recurrence risk in both subtypes of PTB when compared to their non-Hispanic White counterpart.

Conclusion: We observed significant disparities in recurrent PTB by their subtypes and maternal race/ethnicity among a large integrated healthcare system in Southern California. Identifying at-risk populations based on PTB subtypes may be particularly important for intervention strategies targeting sPTB.