Trends, seasonality and effect of ambient temperature on preterm delivery

To identify trends in preterm delivery (PTD) as well as seasonality, temporal variation and the effect of heat stress on its incidence.

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.

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.

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.