The association between ambient temperature and preterm birth in Shenzhen, China: a distributed lag non-linear time series analysis

Background: A few studies have examined the association between ambient temperature and preterm birth (PTB), and the results have been inconsistent. This study explored the association between ambient temperature and PTB in Shenzhen, China. Methods: Data of daily singleton PTB, air pollution and meteorological variables from 2005 to 2011 were collected in Shenzhen. A distributed lag non-linear model (DLNM) was used to investigate the association of the low and high temperatures (1st, 5th, 95th, and 99th percentiles) with PTB. Results: The median temperature was 24.5 degrees C and the 1st, 5th, 95th, and 99th percentiles of daily mean temperatures were 9, 12.5, 29.9 and 30.7 degrees C, respectively. The prevalence of singleton PTB was 5.61 % in Shenzhen. The association between temperature and PTB was not linear. There was an immediate positive association of low temperature (1st and 5th percentiles) and a negative association of high temperature (95th and 99th percentiles) with PTB. The effect of low temperature 9 degrees C (1st) on PTB on the current day was stronger than that of 12.5 degrees C (5th), with a relative risk (RR) of 1.54 (95 % CI: 1.36-1.75) and 1.49 (95 % CI: 1.35-1.63), respectively. The cumulative RR (up to 30 days) of 9 and 12.5 degrees C was 1.72 (95 % CI: 1.28-2.33) and 1.96 (95 % CI: 1.60-2.39), respectively. The cumulative effects (up to 30 days) of high temperature (95th and 99th percentiles) on PTB were 0.69 (95 % CI: 0.60-0.80) and 0.62 (95 % CI: 0.52-0.74), respectively. The cumulative effect (up to 30 days) of low temperatures on vaginal delivery PTB was lower than that of the cesarean section PTB with an RR of 1.58 (95 % CI: 1.12-2.22) and 1.93 (95 % CI: 1.21-3.08), respectively. Conclusions: This study suggests that low temperature might be a risk factor, while high temperature might be a protective factor of PTB in Shenzhen.