Trends, seasonality and effect of ambient temperature on preterm delivery.

Purpose: To identify trends in preterm delivery (PTD) as well as seasonality, temporal variation and the effect of heat stress on its incidence.
Materials and Methods: In this retrospective population-based study, we included all deliveries taking place at the Soroka University Medical Center between the years 1988-2012. A time series database was built including meteorological factors and the number of spontaneous versus induced PTDs for each day. Data were analyzed using time-series analyses.
Results: During the study period, 263 709 deliveries occurred, 7.9% of which were preterm. Spontaneous PTD rate steadily decreased, while induced PTD rate increased. A significant annual and seasonal variation was noted in PTD incidence. A significant higher incidence of spontaneous PTD was demonstrated during the summer period with an incidence rate ratio (IRR) of 4.1 (95%CI: 3.1-5.5; p < 0.001). This trend was not significant for induced PTD. A significant linear association was noted between the heat-stress index and the rates of spontaneous (IRR = 1.07, 95%CI: 1.05-1.10; p < 0.001) but not induced PTDs.
Conclusions: Spontaneous PTD is more common during the summer and its rate is declining steadily over the past decades. Increased outdoor temperature has a significant effect on the incidence of spontaneous, but not induced, PTD.