Introduction: Aim of this study is to compare the lung function variables, in different trimesters of pregnancy. Materials and Methods: Pregnant women from the antenatal clinic of Government Hospital, Kilpauk Medical College, Chennai -10, were selected for this study. The study group consists of 107 women, of whom 18 women in the first trimester, 20 women in the second trimester, 45 women in the third trimester, and 24 women were in control group. Result: Results showed FVC in all the three trimesters are less in comparison to controls. The decrease in FVC values is not statistically significant in first and second trimester but highly significant results were observed in third trimester when compared to controls. Conclusion: FEF 25 -75, a small airway function showed a significant increase in first trimester pregnant women when compared to controls.