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. 2020 Nov 4;371:m3811. doi: 10.1136/bmj.m3811

Table 1.

Meta-analysis results by outcome and temperature metric

No of studies Synthesis method No of studies in summary measure Average effect size
(OR (95% CI))
I2 (%)
Preterm birth:
 Odds of preterm birth during heatwaves 6 Meta-analysis 6 1.16 (1.10 to 1.23) 44.7
 Odds of preterm birth per 1°C temperature increase 6 Meta-analysis 6 1.05 (1.03 to 1.07); 5% increase per 1°C rise (3% to 7%) 87.7
 Odds of preterm birth during high versus low temperatures (exposure over a trimester or all of pregnancy) 9 Meta-analysis 9 1.14 (1.11 to 1.16) 88.2
8* 1.15 (1.13 to 1.18) 65.2
 Odds of preterm birth during high versus low temperatures with exposure period <4 weeks 21 Meta-analysis 21 1.01 (1.01 to 1.02) 89.8
19† 1.05 (1.04 to 1.05) 83.6
21 Summary of effect estimates 21 Median OR=1.07 (IQR 1.05-1.16; range 0.99-1.94)
Birth weight:
 Odds of low birth weight during high versus low temperatures (exposure over a trimester or all of pregnancy) 9 Summary of effect estimates 8‡ Median OR 1.09 (IQR 1.04-1.47; range 1.01-2.49)
 Changes in birth weight at high versus low temperatures 6 Summary of effect estimates Median difference −25.5 g (range −39.4 to −15.0)
Stillbirth:
 Odds of stillbirth during heatwaves 1 1 1.46 (1.09 to 1.96)
 Odds of stillbirth per 1°C temperature increase 3 Meta-analysis 3 1.05 (1.01 to 1.08) 81.3
 Odds of stillbirth during high versus low temperatures (exposure over a trimester or all of pregnancy) 2 Meta-analysis 2 3.39 (2.33 to 4.96) 27.8
 Odds of stillbirth during high versus low temperatures with exposure period <1 week 4 Meta-analysis 4 1.24 (1.12 to 1.36) 53.1

IQR=interquartile range; OR=odds ratio.

*

Excludes Liang (2016).46

Excludes Lee (2008)47 and Liang (2016).46

MacVicar (2017)48 reported a non-significant association, but effect estimates not provided.

§

Madsen (2010)49 and Bruckner (2014)50 reported non-significant associations, but effect estimates not provided.