Prenatal exposure to fine particulate matter, maternal hemoglobin concentration, and fetal growth during early pregnancy: associations and mediation effects analysis

Background: Fetal essential organ development is completed during early pregnancy which is important for fetal and postnatal health. However, the effect of exposure to PM2.5 on fetal growth during early pregnancy is less studied and the related mechanisms are largely unknown. Methods: We conducted a birth cohort study of 1945 pregnant women with measurement of the fetal crown to rump length (CRL) by ultrasound between the gestational age of 11 and 14 weeks. We estimated residential exposures of PM2.5 from the date of LMP to the date of ultrasound examination using a spatial-temporal land use regression model. Maternal hemoglobin concentration was examined by maternal blood samples during the same gestational period or +/- 1 week of the ultrasound examination. The associations of exposure to PM2.5 with maternal hemoglobin concentration, and exposure to PM2.5 with fetal CRL during early pregnancy were estimated by multiple linear regression models. The mediation effect of maternal hemoglobin concentration on the association between exposure to PM2.5 and fetal CRL was explored by a casual mediation analysis. Results: One IQR increment of prenatal exposure to PM2.5 was associated with a 0.929 g/L (95% CI: 0.068, 1.789) increase in maternal hemoglobin concentration, and associated with a - 0.082 cm (95% CI: 0.139, - 0.025) decrease in fetal CRL. One g/L increment of maternal hemoglobin concentration was associated a - 0.011 cm (95% CI: 0.014, - 0.008) decrease in fetal CRL. The mediation analysis indicated that 12.1% of the total effect of prenatal exposure to PM2.5 on reducing fetal CRL was mediated by increased maternal hemoglobin concentration. Conclusion: Exposure to PM2.5 was associated with reduced fetal growth during early pregnancy and elevated maternal hemoglobin concentration mediated this association.