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Critical windows and joint effects of prenatal heat and relative humidity exposure on stillbirth: A prospective
Zhan Li1, Yuxin Bi2, Shuangying Li1
1Department of Central Laboratory, Beijing Obstetrics and Gynecology Hospital, Capital Medical University, Beijing Maternal and Child Health Care Hospital, Beijing, 100026, China.
Objective:
To identify critical windows and explore joint effects of prenatal heat and relative humidity (RH) exposure on stillbirth risk.
Study:
DesignProspective longitudinal cohort study.
Setting:
Based on the China Birth Cohort Study (CBCS) covering 334,924 participants enrolled between February 2018 and December 2022 among 52 hospitals in mainland China.
Population:
243,262 live births and 942 stillbirths.
Methods:
A distributed Lag Nonlinear Model (DLNM) with time-dependent Cox regression models was used to estimate the relationship between weekly maternal exposure to heat and RH and the risk of stillbirth. The relative excess risk due to interaction (RERI) was used to assess whether there was a deviation from the additive joint effect.
Main Outcome Measures:
Stillbirth.
Results:
The association between humidity and stillbirths was U-shaped, with the lowest incidence observed at 55%. Critical susceptible windows were found during 32nd -36th gestational weeks, with the strongest hazard ratio (HR) in the 35th week for stillbirth at 90th centile heat exposure and during gestational weeks 27th -33rd with the strongest HR in the 32nd week at 90th centile RH exposure. Heat and high RH (≥55%) exposure showed a less-than-additive interaction, with high humidity attenuating the heat-related risk, resulting in an antagonistic interaction on the additive scale.
Conclusions:
The second and third trimesters, and especially the third trimester, are critical periods of susceptibility to the effects of heat and RH on stillbirths. Dry heat has a higher risk of stillbirth than wet heat. Given the influence of global warming, our findings add new evidence to understand the exposure to heat and RH in stillbirths.
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