Migration, Duration of Residence and Spontaneous and Iatrogenic Preterm Birth: A Retrospective Cohort Study

OBJECTIVE: To investigate whether the 'healthy migrant' effect applies to PTB and its subtypes, iatrogenic (iPTB) and spontaneous (sPTB) and whether this association differs by length of residence. DESIGN: Cohort study using electronic health records. SETTING: New South Wales, Australia. POPULATION: Singleton births between June 2020 and November 2023. METHODS: Migrant status was categorised as Australian-born, new migrants (< 5 years residence) or established migrants (≥ 5 years residence). Logistic regression estimated adjusted odds ratios (aORs) for associations between migrant status and PTB, controlling for demographic, social and service factors. MAIN OUTCOME MEASURES: Odds of PTB, iPTB and sPTB reported as aORs with 95% confidence intervals. RESULTS: Among 60 806 births, 4076 (6.7%) occurred preterm. Compared to Australian-born women, both new migrants (aOR 0.74, 95% CI 0.65-0.85) and established migrants (aOR 0.84, 95% CI 0.75-0.93) had lower odds of PTB, largely attributable to lower odds of iPTB (new migrants aOR 0.68, 95% CI 0.57-0.83; established migrants aOR 0.80, 95% CI 0.69-0.92). Lower odds of sPTB were observed only among new migrants after full adjustment (aOR 0.80, 95% CI 0.65-0.97). There was evidence that the association between migrancy duration and PTB varied by area-level socioeconomic status, with lower odds of PTB among migrants compared with Australian-born women most apparent in the most disadvantaged areas. CONCLUSIONS: These findings may be consistent with a 'healthy migrant' effect for PTB, with lower odds observed among both new and established migrants and a larger reduction among new migrants. Lower odds of iPTB were observed among both migrant groups, whereas lower odds of sPTB were observed only among new migrants. Differences in baseline health, obstetric decision-making and health-system engagement may contribute to these observed associations.