Drops in Atmospheric Pressure and Subsequent Fluctuations in Daily Delivery Volume: A Case‐Crossover Study

Fluctuations in daily delivery volume impact obstetricians' workload and maternal-fetal complications. While sudden drops in atmospheric pressure (e.g. due to typhoons or hurricanes) may be associated with an increase in daily delivery volume in the following days, few studies have explored these associations by gestational week while considering lagged effects. To investigate whether drops in atmospheric pressure are associated with fluctuations in the daily volume of deliveries with spontaneous onset of labour. We employed a time-stratified case-crossover design. Data on the daily volume of spontaneous-onset deliveries from 47 prefectures between 2011 and 2020 were obtained from the Japan Perinatal Registry Network database. Quasi-Poisson regression models with distributed lag nonlinear models were used to examine the association between pressure drops (e.g. -13.8 hectopascal (hPa), representing the 1st percentile) and daily delivery volume in each prefecture, with lag periods of up to 14 days. Prefectural estimates were combined using random-effects meta-analysis. Stratified analyses were conducted by the gestational week, categorised as < 37, 37, 38, 39, 40 and 41+ weeks. A total of 1,074,380 spontaneous-onset deliveries were recorded. The lag-cumulative relative risk of spontaneous-onset deliveries peaked with a lag period of 0-4 days and became negligible beyond 1 week, likely due to harvesting effects. The associations were most pronounced among women with a gestational age between 38 and 40 weeks. Specifically, for a PCN of -13.8 hPa, the 0-4 day lag-cumulative relative risk was as follows: at 38 weeks, 1.07 (95% confidence interval [CI] 1.00, 1.14); at 39 weeks, 1.08 (95% CI 1.02, 1.14); and at 40 weeks, 1.10 (95% CI 1.03, 1.19), compared to no pressure change. Pressure drops were associated with a modestly larger volume of spontaneous-onset deliveries over the following few days, especially at full-term gestation.