Temporal trends in late-pregnancy exposure to ambient temperature and risk of preterm birth in Japan, a nationwide study from 1979 to 2023.

BACKGROUND

Evidence linking heat exposure with preterm birth (PTB) has grown amid climate change. However, evidence remains limited across the temperature spectrum or temporal changes of susceptibility.

METHODS

To assess the temporal and spatial variation in the short-term association between ambient temperature and PTB in Japan from 1979 to 2023, we applied a two-stage design using a time-stratified case-crossover and conditional Poisson model incorporating a 28-day distributed lag non-linear model. Prefecture-specific estimates were pooled using random-effects meta-analysis. To explore temporal changes in susceptibility, the data were split into four decadal subperiods, and mixed-effects meta-regression examined prefecture-level predictors of changing risk.

FINDINGS: Between 1979 and 2023, 2,604,733 singleton PTBs (4.9% of 52,749,360 live births) occurred. Compared to the minimum morbidity temperature, 17.7 °C (56th percentile), relative risks (RRs) were 1.13 [95% CI: 1.10-1.15] for heat (99th percentile) and 1.11 [95% CI: 1.09-1.14] for cold (1st percentile). Nationally, 4.7% [4.3-5.0%] of PTBs (102,266 cases) were attributable to non-optimal temperatures (1.9% heat; 2.8% cold). Heat-related RR declined from 1.16 [1.12-1.21] in 1979-1989 to 1.07 [1.03-1.12; p: 0.0084] in 2001-2011 and then plateaued, while cold-related risk remained stable. Air conditioning prevalence was positively associated with MMT across periods.

INTERPRETATION: Over four decades, Japan experienced modest early declines in heat-related risk, with no further improvement in recent decades, alongside persistent cold-related PTB risk, suggesting limited heat adaptation. Pregnant women remain underrepresented in climate adaptation planning, underscoring the need for gender-inclusive resilience strategies.

FUNDING: Japan's Environmental Restoration and Conservation Agency.