Seasonal patterns of preterm birth in industrialised countries and the effect of weather

Background: Preterm birth, or birth before 37 completed weeks of gestation (259 days), is reported to be responsible for the majority of neonatal deaths and accounts for 5-10% of all births in developed countries. This study was undertaken to establish whether a yearly pattern of preterm births in London exists and if so, how this pattern might lend itself to the interpretation of possible etiologic factors at the broader level of environmental exposures.

Methods: A literature review was undertaken to identify any previous studies in which a seasonal pattern of preterm birth was reported. The yearly preterm birth pattern in London, UK was determined using data from 495,378 singleton deliveries from the years 1988 to 2000. Any pattern in the proportion of preterm births exhibited by this population was compared against those from studies located in the literature review. The London data was then investigated for possible associations with daily mean temperature, daily amount of rainfall, daily hours of sunshine, daily mean barometric pressure, the largest daily drop in barometric pressure and daily mean relative humidity. Lags of up to three months were analysed.

Results: Previous reports revealed that preterm birth rates in Japan and the USA were at their highest twice a year, resulting in a yearly pattern with one peak in summer (Jun-Aug) and one in winter (Dec-Jan). Preterm birth proportions in the London-based population peaked only once a year in winter (Dec-Jan). Contrary to patterns in Japan and the USA, the lowest proportions of preterm birth were observed in the summer (Jun-Aug).

Proportions of preterm birth increased with mean daily barometric pressure above 1030mb or below 995mb. Increasing temperature had an inverse association with preterm birth proportions, as did increasing hours of sunshine and decreasing levels of humidity. However, due to seasonal associations between temperature, humidity and hours of sunshine the effect on preterm birth could not be interpreted independently. The proportion of preterm births showed no apparent association with the daily amount of rainfall or the largest daily drop in barometric pressure. Results of time-series analysis, controlling for potential confounding due to seasonality will be discussed.

Conclusions: While rates of preterm birth in countries such as Japan and the USA are at their highest in summer and winter, preterm births in this London-based population peak only once a year in winter. This difference may be due to the lack of extreme temperatures in this region, rather than a protective effect. The determination of seasonal patterns in preterm birth is essential to effective service planning and delivery and elucidating any correlations between weather variables and preterm birth may help efforts to reduce preterm birth and its consequences.