Background: The body position may have an effect on gas exchange though the magnitude of this effect has not been studied thoroughly during the three trimesters of pregnancy. Objectives: This study analyzes the effect of body position change on arterial oxygen tension (PaO₂) and arterial carbon dioxide tension (PaCO₂) in healthy pregnant women. Methods: We examined 21 lung healthy pregnant women at the end of the first, second and third trimester of pregnancy. Blood was drawn from the radial artery first in the sitting and subsequently in the supine position. Peak expiratory flow rate was determined as well. Results: No statistically significant difference was detected between the mean values of PaO₂ in the supine and in the sitting position at the end of the first trimester. On the contrary, at the end of the second trimester the mean value of PaO₂ in the supine position was 92.39 ± 1.95 mm Hg, while in the sitting position the mean PaO₂ was 96.56 ± 1.83 mm Hg (p < 0.001). At the end of the third trimester the mean PaO₂ in the supine position was 90.48 ± 2.91 mm Hg while in the sitting position the mean PaO₂ was 97.48 ± 1.46 mm Hg (p < 0.001). Conclusions: According to our data we conclude that the statistically significant difference in PaO₂ in the sitting and supine positions clearly shows that the position needs to be considered both when attempting to establish reference values and when evaluating gas tensions at the end of pregnancy.