| Background and objective:In recent years,the occurrence of preterm birth has been increasing.Preterm birth has become one of the great public health challenges facing the world.Preterm birth may be affected by many factors.With the impact of the environment on health in recent years,people’s attention has gradually increased.The studies of the impact of the environment on preterm birth focuses on two aspects.One is the effect of extreme ambient temperature on preterm birth.The second is the impact of ambient PM2.5 exposure on preterm birth.At present,the relationship between ambient temperature and preterm birth is not clear,and there is no study on the relationship between exposure to ambient temperature and preterm delivery in different periods of pregnancy in China.Research on the relationship between ambient PM2.5 exposure levels and preterm birth is still rare,and the relevant mechanisms are not yet clear.The purpose of this study was to investigate the effect of exposure to extreme ambient temperature and the level of PM2.5 exposure during pregnancy on premature birth,and to provide basis for the formulation of health policies and the care of pregnant women.Subjects and methods:Investigated the baseline data and pregnancy outcomes of 1,020,471 pregnant women from 132 cities in China participating in the National Free Pre-Pregnancy Health Check Project between 2010 and 2013.Daily average temperature exposure data for each city was collected on the National Weather Service website and matched to the maternity data based on the address.Temperature exposure windows were divided into different periods(3 months before pregnancy,1-7 weeks of pregnancy,8-14 weeks of pregnancy,15-21 weeks of pregnancy,1 week before delivery,4 weeks before delivery,and entire pregnancy).According to the local average temperature,the study area was divided into three categories(cold,moderate,hot)through cluster analysis.Extreme ambient temperature is defined as extremely cold(temperature exposure below the local 5th percentile)and extreme heat(temperature exposure above the local 95th percentile).Multivariate logistic regression was used to analyze the effect of extreme temperatures exposure on preterm birth at different periods and in different regions.This study also collected information on 426,246 pregnant women who participated in the National Free Pregnancy Health Check Project from January 2014 to December 2014 and had follow-up results of pregnancy outcomes.The data of the daily ambient PM2.5 exposure levels of each city was collected through the National Environment Monitoring Center website and matched with the maternal data according to the address information.The subject’s ambient PM2.5 exposure period was divided into the first trimester(1-13 weeks of pregnancy),the second trimester(14-26 weeks of gestation),the third trimester(27-37 weeks of pregnancy)and the entire pregnancy.The average ambient PM2 5 exposure levels during different pregnancy periods and throughout pregnancy were calculated by the average daily PM2.5 exposure level data.A Cox proportional hazards regression model was used to analyze the relationship between PM2.5 exposure levels and preterm birth during each exposure period.Results:1.A total of 1,020,471 pregnant women with a single live birth outcome were included,of whom 73240(7.2%)were preterm birth.Compared with the medium level(temperature exposure is between the local 5th to 95th percentile),extreme hot exposure during different gestation periods increased the risk of preterm birth in hot areas,with the most significant increase during 3 months before pregnancy(OR=1.23,95%CI:1.17-1.30).Contrary to hot exposure,extremely cold exposure in hot areas reduced the risk of preterm birth,with the most significant protective effect during 3 months before pregnancy(OR=0.78,95%Cl:0.73-0.83).In cold and moderate areas,extremely cold exposure also reduced the risk of preterm birth.The analysis of the relationship between the level of exposure to various ambient temperatures and preterm birth during the entire period of pregnancy also showed that prolonged exposure to extremely hot conditions throughout the pregnancy in hot areas increases the risk of preterm birth.2.A total of 426,246 pregnant women with single birth outcome Mwere included,of whom 35,261(8.3%)were preterm.The effect of an increase in PM2.5 of 10 μg/m3 on preterm birth was most pronounced in the third trimester(HR=1.06,95%CI:1.06-1.07).PM2.5 exposure during the first trimester(HR=1.04,95%CI:1.03-1.04),the second trimester(HR=1.02,95%CI:1.02-1.02)and the entire pregnancy(HR=1.06,95%CI:1.05-1.06)also increased the risk of preterm birth.Compared with the lowest quartile,the high quartiles of ambient PM2.5 exposure significantly increased the risk of preterm birth and was most significant in the third trimester(HR=1.87,95%CI:1.69-2.06).The results of subgroup analysis showed that women who were older,low education levels,occupational farmers,formerly pregnant,not living in coastal areas,and pregnant in winter had a higher risk of preterm birth that associated with PM2.5 exposure.Pregnant women living in moderate and hot temperature area were more affected by ambient PM2.5 exposure.Conclusions:1.The results of this study showed that pregnancy outcomes of women of childbearing age were affected by ambient temperature,short-term and long-term exposure to extreme ambient temperatures before and during pregnancy may affect the risk of preterm birth.Exposure to extreme hot conditions is a risk factor for preterm birth and exposure to extreme cold conditions may be a protective factor for preterm birth.2.Ambient PM2.5 exposure level is positively associated with the risk of preterm birth.This association is most pronounced during the third trimester.Women who are older,less educated,occupationally farmer,formerly pregnant,not living in coastal areas,and pregnant in winter have a higher risk of preterm birth associated with exposure to ambient PM2.5.Appropriate measures should be taken to reduce the exposure of the pregnancy women to ambient air pollution. |
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