Temporal Trends in Preterm Birth Associated With Hypertensive Disorders of Pregnancy in Victoria, Australia: A...

Hypertensive disorders of pregnancy (HDP) are a leading reason for medically indicated preterm birth (PTB), yet their contribution to population-level PTB trends is poorly understood. We examined temporal trends in PTB overall and HDP-associated PTB in Victoria over an 11-year period spanning the COVID-19 pandemic, and distinguished changes in HDP prevalence from changes in preterm delivery among affected pregnancies.

Population-based interrupted time-series study using seasonal autoregressive integrated moving average (SARIMA) models, with July 2018 (launch of a national preterm birth prevention program) pre-specified as a temporal reference point.

All hospitals in Victoria, Australia.

779,325 singleton births at ≥ 20 weeks' gestation between 1 January 2012 and 31 March 2022.

Monthly rates (per 1000 singleton births) of overall PTB, HDP and HDP-associated PTB. Decomposition analyses separated changes in HDP prevalence from changes in preterm delivery among affected pregnancies.

Overall PTB was lower after July 2018 (5.8%-5.6%), while HDP prevalence increased (6.4%-7.0%). HDP-associated PTB increased before mid-2018 (+0.75 per 1000 births per month; 95% confidence interval [CI], 0.50-1.00) but declined thereafter (-1.12 per 1000 births per month; 95% CI, -1.88 to -0.36). This reversal was driven predominantly by reduced iatrogenic PTB with HDP; spontaneous PTB with HDP showed no significant change. Despite rising HDP prevalence, decomposition analyses estimated 89 fewer HDP-associated PTB cases than expected under pre-2018 patterns, reflecting a decline in the proportion of HDP pregnancies delivered preterm. PTB without HDP showed no significant temporal change.

PTB among HDP-complicated pregnancies declined despite increasing HDP prevalence, consistent with a shift towards later gestational age at delivery rather than reduced disease incidence. These findings may reflect improvements in care and demonstrate that reductions in iatrogenic PTB are achievable even as major risk factors become more prevalent.