- 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 vs. 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.