Temporal trends and recurrence risks of spontaneous and provider-initiated preterm birth in Canada – a population-based study

BACKGROUND: Rates of preterm birth in Canada are increasing; however, national reports typically only include aggregate preterm birth rates and do not provide information on sub-types of preterm birth. This study aimed to describe temporal trends in preterm premature rupture of membranes (PPROM), spontaneous preterm labor, and provider-initiated preterm birth in Canada. METHODS: Hospitalization data on livebirths in Canada (excluding Quebec) between 2004/05-2024/25 were obtained from the Canadian Institute of Health Information Discharge Abstract Database. Cochran-Armitage tests for trend were used to examine temporal trends in preterm birth (overall and by sub-type) per 100 live births. For primiparous females with ≥2 deliveries, a recurrence matrix was calculated to examine the probability of recurrent preterm birth. RESULTS: Amongst 5 640 416 deliveries 7.4% were preterm, of which 39.8% were provider-initiated and 60.2% were spontaneous (34.0% PPROM, 60.3% spontaneous preterm labor, 5.7% unclassified). The preterm birth rate increased from 7.3% to 7.8%. Significant overall temporal increases masked increases in provider-initiated preterm birth and decreases in spontaneous preterm birth. The probability of recurrent preterm birth was 22.7% following a provider-initiated preterm birth, 25.7% following PPROM, 29.3% following spontaneous preterm labor, and 7.0% if the first delivery occurred at term. CONCLUSION: Preterm birth rates continue to increase in Canada, and a large proportion of this is attributed to increasing rates of provider-initiated preterm birth in the late preterm period. While previous preterm birth remains a risk factor for subsequent preterm birth, not all previous preterm births convey the same probability of recurrence in subsequent pregnancies.