INTRODUCTION:This study was conducted to compare the pulmonary function and the body composition between trimesters of pregnancy and, to determine the variables correlated to the peak expiratory flow (PEF) and the expiratory flow between 25% and 75% of vital capacity (FEF25%75%). METHODS:A cross-sectional study examined 120 healthy low-risk pregnant women in the three trimesters of pregnancy, measured by spirometry and multisegmental electrical impedance. Women between the fifth and 40th gestational week, ages between 18-35 years, singletons, and sedentary were included. Smokers, individuals with cardio-respiratory diseases, neurological and chest deformities were excluded. RESULTS:The vital capacity, the expiratory volume in one second, the peak expiratory flow and expiratory flow between 25% and 75% of vital capacity decreased with the progression of pregnancy, the multiple linear regression analysis showed that height and maternal age were associated with PEF, being responsible for explaining 14.7% of its variability. The prepregnancy weight explained 6.5% of the variability of the FEF25%75%. The multiple linear regression analysis showed that height and maternal age were associated with PEF, explaining 14.7% of its variability. CONCLUSION:This study showed that the volumes and lung capacity in healthy pregnant women do not seem to change with the progress of pregnancy. The pre-pregnancy weight was related to the FEF25%75%. Height and maternal age had greater influence on PEF changes, while the body composition and obstetric variables did not influence it.