Associations Between Prenatal Exposure To Ambient Temperature And Placental Morphological Changes: A Prospective Cohort Study

Background:Placenta is an important organ for the exchange of substances between mother and fetus during human pregnancy.It has functions of nutrition transport,waste metabolism,defense barrier,and synthesis.Fetal growth and development depend on a healthy and well-functioning placenta.Placental weight,volume,and placental weight/birth weight ratio(PFR)are important morphological indicators to measure the growth of the placenta.Studies have shown that the change of placental weight,volume and PFR were associated with the adverse birth outcomes.Therefore,it is important to explore the reason for placental morphological changes to reduce the risk of adverse birth outcomes.In recent years,animal experimental studies have suggested that prenatal exposure to extreme temperatures were associated with the reduction in placental weight and volume.However,epidemiological evidence based on population studies was still lack.Secondly,the exposure windows used to evaluate the effect of ambient temperature on the placenta were not clear.Accurate small-scale exposure windows such as a gestational month or gestational week were less used.Finally,many studies have used the data from meteorological monitoring stations to evaluate temperature exposure instead of individual exposure level,the studies used geographic information combined with mathematical models to evaluate temperature exposure level were limited.Objectives:Based on the prospective cohort study,we aimed to estimate the associations between prenatal exposure to ambient temperature with placental morphological indicators,and to explore the possible exposure windows of temperature on the placenta.Methods:This study was based on the Prenatal Environments and Offspring Health(PEOH)cohort study,which was conducted in Guangzhou Panyu Central Hospital in 2016.Women which received antenatal care and delivery in the hospital were recruited in the cohort study.A total of 4,051 participants were finally included in our study.We have used the t-test and analysis of variance to test the differences in the distribution of outcomes in general characteristics of participants.Pearson correlation analysis was used to estimate the correlation between placental morphological indicators and the average level of temperature,relative humidity,and pollutants during pregnancy.The generalized additive models were used to estimate the associations between ambient temperature exposure and placental indicators and to further evaluate the sensitive exposure window.We set the 20℃as the reference temperature.The 95th centile(29℃)and the 5th centile(15℃)of temperature exposure levels were used as high-temperature and low-temperature exposure for all subjects,and we have reported their effects on placental morphological indicators.Results:A total of 4051 participants were included for analysis,and the mean(standard deviation)of placental weight,volume,and PFR of pregnant women were 520.2 g(54.5 g),717.1 cm~3(110.8 cm~3),and 16.7(2.4),respectively.The maximum,mean,and minimum values of temperature during pregnancy were 30.3℃,14.1℃,and 23.4℃,respectively.Compared with the mild temperature,the pregnant women’s exposure to high temperature during late pregnancy was associated with a6.03 g(95%confidence interval[CI]:11.28 g,0.78 g)decrease of placental weight.The pregnant women high temperature exposure during 29-32weeks,33-36 weeks,37-40 weeks and late pregnancy were associated with18.26 cm~3(95%CI:-31.51 cm~3,-5.02 cm~3),14.20cm~3(95%CI:-24.21cm~3,-4.19cm~3),19.34cm~3(95%CI:-30.99cm~3,-7.69cm~3)and 16.15cm~3(95%CI:-26.24cm~3,-6.07cm~3)reduction of placental volume,respectively.Maternal exposure to high temperature at 25-28 weeks,29-32 weeks,33-36weeks,and the late pregnancy was associated with 0.35(95%CI:0.04,0.66),0.28(95%CI:0.01,0.54),0.19(95%CI:0.05,0.33),and 0.26(95%CI:0.07,0.45)increases in PFR,respectively.We did not find a significant association between low temperature(15℃)exposure during pregnancy and placental morphological indicators.Conclusions:(1)The pregnant women’s exposure to high temperature was associated with the reduction of placental weight and volume,and with an increase of PFR.(2)The late pregnancy might be the critical exposure window for the effects of high temperature on placental morphological indicators.