BACKGROUND AND AIM: The airway macrophages carbon loading (AMCL) has been suggested to be a biomarker of the long-term exposure to air pollution; however, to date no study has characterized AMCL for the pregnancy period. Therefore, this study aimed to assess the determinants of AMCL during pregnancy in Iran, a middle-income country. METHODS: This study was based on a sample of 234 pregnant women with term and normal vaginal delivery who were residing in Sabzevar, Iran (2019). Image analysis was applied to calculate the carbon area (µm2) in airway macrophages as an indicator of AMCL. Measuring the AMCL was performed by two independent investigators and checked by third investigator to reduce the AMCL measurement bias. We characterized 35 potential determinants of personal exposure to air pollution for each participant, including six personal, nine indoor, and 20 home-outdoor factors. We applied Deletion/Substitution/Addition algorithm to identify the most relevant determinants that could predict AMCL levels. RESULTS: The median (IQR) of AMCL level was 0.12 (0.30) µm2 with a successful sputum induction in 82.9% (194) of participants. Ambient residential PM2.5 levels were positively associated with higher AMCL levels. On the other hand, increased residential distance to the traffic lights, squares and ring-roads, the duration of opening window per day, and opening window during cooking were inversely associated with AMCL levels. CONCLUSIONS: This study is the first to characterize AMCL during pregnancy. Moreover, this is one of the largest reported studies on characterization of AMCL levels. Furthermore, this is the first study to explore the personal, indoor and outdoor determinates of personal exposure to air pollution in pregnant women in an LMIC. Our findings provide novel insights on the different personal, indoor, and outdoor determinants of personal exposure to air pollution during pregnancy in a middle-income country. KEYWORDS: Air pollution, Pregnancy, Biomarker, Lung