Association between low ambient temperature during pregnancy and adverse birth outcomes: a systematic review and meta-analysis

Abstract

Background: Extreme temperature events, including extreme cold, are becoming more frequent worldwide, which might be harmful to pregnant women and cause adverse birth outcomes. We aimed to investigate the association between exposure to low ambient temperature in pregnant women and adverse birth outcomes, such as preterm birth, low birth weight, and stillbirth, and to summarize the evidence herein. Methods: Relevant studies were searched in PubMed, Cochrane, and Embase electronic databases until November 2021. Studies involving low ambient temperature, preterm birth, birth weight, and stillbirth were included. The guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-analyses were followed to conduct this study risk of bias and methods for data synthesis.Results: A total of 34 studies were included. First, pregnant women exposed to low ambient temperature had an increased risk of preterm birth (risk ratio [RR] 1.08; 95% confidence interval [CI] 1.04-1.13). Subgroup analyses revealed that exposure during late pregnancy was more likely to induce preterm birth. In addition, only pregnant women exposed to <1st percentile of the mean temperature suffered increased risk of preterm birth. Moreover, pregnant women living in medium or hot areas were more prone to have preterm births than those in cold areas when exposed to low ambient temperatures. Asians and Blacks were more susceptible to low ambient temperatures than Caucasians. Second, pregnant women exposed to low ambient temperature had an increased risk of low birth weight (RR 1.07; 95% CI 1.03-1.12). Third, pregnant women had an increased risk of stillbirth while exposed to low ambient temperature during the entire pregnancy (RR 4.63;. Conclusions: Exposure to low ambient temperature during pregnancy increases the risk of adverse birth outcomes. Pregnant women should avoid exposure to extremely low ambient temperature (<1st percentile of the mean temperature), especially in their late pregnancy. This study could provide clues for preventing adverse outcomes from meteorological factors. Registration: No. CRD42021259776 at PROSPERO (https://www.crd.york.ac.uk/PROSPERO/).

Ambient temperature and preterm birth: A case‐crossover study

“…Our findings on the association of low temperature and increased risk of PTBs are consistent with a previous review that included 34 studies. 8 For example, a study conducted in Guangzhou, China, involving 838 146 singleton vaginal births, revealed that exposures to extreme cold (the 1st percentile, 7.6°C) during the last 4 weeks of pregnancy were associated with a 17.9% (95% CI 10.2-26.2) increased risk of PTBs. 45 A meta-analysis of nine studies confirmed a 14% (95% CI 6-23) increased risk of PTBs associated with low (<1st percentile) temperature, 8 which is comparable to our findings.…”

“…8 For example, a study conducted in Guangzhou, China, involving 838 146 singleton vaginal births, revealed that exposures to extreme cold (the 1st percentile, 7.6°C) during the last 4 weeks of pregnancy were associated with a 17.9% (95% CI 10.2-26.2) increased risk of PTBs. 45 A meta-analysis of nine studies confirmed a 14% (95% CI 6-23) increased risk of PTBs associated with low (<1st percentile) temperature, 8 which is comparable to our findings. Even though people dress warmly in cold weather, exposure to low temperatures, for example on the face, can lead to vasoconstriction, 46 potentially affecting PTB by reducing uteroplacental blood flow.…”

“…13 While recent systematic reviews have indicated possible associations between low-and high-temperature exposures and increased PTB risks, they have also reported inconsistencies in the findings across studies. 7,8 Many of these studies were conducted in a single city, which may introduce selection biases. 14 Although some nationwide studies have aggregated region-specific risks in the USA 15 and China, 14 regional heterogeneity on access to maternal and child health…”

“…Among the environmental factors that may affect PTB, ambient temperatures, both low and high, have been identified as potential contributors. 7,8 Low temperatures may induce increased blood viscosity and vascular constriction, 9 ultimately impairing uteroplacental blood flow and potentially resulting in PTB. 10,11 In addition, cold-stress may trigger the release of placental corticotropin-releasing hormone, a key factor in initiating human parturition.…”

“…While recent systematic reviews have indicated possible associations between low‐ and high‐temperature exposures and increased PTB risks, they have also reported inconsistencies in the findings across studies 7,8 . Many of these studies were conducted in a single city, which may introduce selection biases 14 .…”