Impact of extreme ambient temperatures on low birth weight: Insights from empirical findings in Pakistan.

BACKGROUND

Exposure to extreme ambient temperatures during pregnancy, including both heat and cold, can lead to complications such as preterm births, low birth weight (LBW), and developmental anomalies. These exposures pose immediate health risks to both mother and child and may exacerbate health disparities across future generations.

OBJECTIVE

Pakistan, with limited health resources, is particularly vulnerable to the impacts of extreme temperatures. This study aimed to quantify the association between heat and cold exposure and LBW in Pakistan.

DESIGN

Space-time-series study design.

METHODS

We analysed 17,077 birth records from 10 datasets from the Multi-Indicator Cluster Surveys and 1 from the Pakistan Demographic and Health Surveys, covering monthly LBW cases from January 2008 to December 2017. These data were linked to monthly heat index estimates, derived from temperature and humidity, from Copernicus ERA5-Land, aggregated at the provincial level. We used a space-time-series study design with quasi-Poisson distributed lag nonlinear regression. Models were adjusted for long-term trends, seasonality, and socio-economic factors, including maternal education, wealth index and rural residence. We estimated the cumulative risk of LBW associated with heat and cold, individual lag effects and the attributable fraction of LBW cases due to temperature exposure.

RESULTS

LBW was reported in 26.02% ( = 4444) of total birth records. The overall exposure-response relationship indicated a positive association between LBW and extreme heat; however, the estimates were imprecise and included the null. At lag 0 (month of conception), there was evidence of increased risk during periods of moderate heat (90th percentile: relative risk (RR) 1.70; 95% confidence interval (CI): 1.01, 2.87) and extreme heat (99th percentile: RR 1.93; 95% CI: 1.00, 3.71). The heat-related attributable fraction for LBW ranged from 0.34 to 0.42 across provinces. In contrast, no association was found between LBW and cold exposure.

CONCLUSIONS

This study contributes to the existing body of evidence of the association between extreme temperatures and LBW, particularly from a low-resource, highly vulnerable country. Notably, we found a positive association between heat exposure and LBW during the first month of pregnancy (lag 0), suggesting that early gestation may be a critical period of vulnerability.