Impact Of PM2.5 Exposure Of Rural Women During Whole Pregnancy On Thyroid Hormones In Newborn Cord Blood

Objectives:The purpose of this study is to investigate the fine particle matter（PM2.5）exposure during whole pregnancy of rural women,the cord blood thyroid hormones levels of there newborn,and explore the impact of the exposure to PM2.5 during pregnancy on fetal cord blood thyroid hormones and identify susceptibility windows.Methods:This study is a prospective birth cohort study,based on the birth cohort study in rural Xuanwei,Yunnan.The total of 716 pregnant women were included in the Xuanwei birth cohort study.In each trimester of pregnancy,the participated women were asked to complete a questionnaire survey and their personal PM2.5 exposures were measured by portable fine particular sensors.We follow up until the fetus was delivered,and the cord blood samples of newborns were collected after delivery,thyroid-stimulating hormone（TSH）and free thyroid hormones（FT3,FT4）in cord blood samples was tested,and complete delivery record.Wilcoxon Z,Kruskal-Wallis H,Spearman correlation analysis,multiple linear regression analysis was used to explore the impacts of exposure to PM2.5 during pregnancy on thyroid hormones levels in cord blood.Results:（1）In this study,93.2%of woman were below 35 years old during their pregnancy.Han mothers accounting for 85.4%,woman received junior high school education and below,accounting for 84.2%.And 45.1%of families had an annual household income of less than RMB 20,000 per year.Women’s BMI before pregnancy in the normal range accounted for 68.5%.The Non-instrumental vaginal delivery was 81.5%.The proportion of boys was 51.7%,the proportion of preterm infant was 5.3%,and born in warm season was 49.4%.（2）The average level of daily PM2.5 exposures in the first,the second,and the third trimesters ranged from 19.68～97.0μg/m³,19.68～71.37μg/m³,19.68～90.64μg/m³respectively.The personal PM2.5 concentrations show significant difference in three trimesters（P<0.05）.The PM2.5 concentration in the first trimester was higher than that in the third trimester.The PM2.5 concentration in the first trimester and the second trimester did not show significant difference.The PM2.5 concentration in the second trimester and the third trimester did not show significant difference.（3）The medians of cord blood TSH,FT3,FT4,FT4/FT3 were 7.78μIU/mL,1.80pmol/L,12.95 pmol/L,7.30 pmol/L.（4）Multiple linear regression analysis revealed that PM2.5 exposure in the first trimester and the second trimester did not show any association with the TSH,FT3,FT4,FT4/FT3 of cord blood after adjusting for other covariates（P>0.05）.The concentration of PM2.5 in the third trimester was negatively correlated with FT4/FT3levels（β=-0.056,95%CI=-0.105～-0.006,P<0.05）.PM2.5 exposure in each trimester was entirely included in the multiple linear regression analysis model indicated that the PM2.5 concentration in the third trimester was negatively correlated with FT4/FT3levels（β=-0.056,95%CI=-0.107～-0.004,P<0.05）.The concentration of PM2.5during pregnancy were divided into low exposure group(<P₂₅),Medium exposure group(P₂₅～P₇₅),and high exposure group(>P₇₅),indicated that high exposure group in the third trimester was negatively correlated with FT4（β=-0.733,95%CI=-1.429～-0.037,P<0.05）,Medium exposure group（β=-1.597,95%CI=-2.719～-0.475,P<0.05）and high exposure group（β=-1.540,95%CI=-2.840～-0.241,P<0.05）in the third trimester were negatively correlated with FT4/FT3 levels.The accumulation of maternal exposure to PM2.5 in whole pregnancy indicated that PM2.5 exposure above WHO guideline in one trimester（β=-1.092,95%CI=-1.915～-0.268,P<0.05）,two trimesters（β=-0.847,95%CI=-1.644～-0.049,P<0.05）,and three trimesters（β=-0.982,95%CI=-1.806～-0.157,P<0.05）were negatively correlated with FT4 level,but not found significant association with TSH、FT3、FT4/FT3 levels.Conclusions:Pregnant women are widely exposed to low PM2.5 daily.The level of PM2.5 exposure in the first trimester was higher than that in the third trimester.PM2.5exposure over 31.38μg/m³ during the third trimester may reduce FT4,FT4/FT3 levels.The PM2.5 exposure over 25μg/m³ only 1 trimester,or both in 2 and in 3 trimesters may reduce FT4 in cord blood,the third trimester may be the critical windows,but didn’t found the daily PM2.5 exposure in the first and the second trimesters affect TSH,FT3,FT4/FT3 levels.