Effects of Maternal Cigarette Smoking on Pulmonary Function, Respiratory Muscle Strength, Functional Exercise Capacity, and Skeletal Muscle Oxygenation During the Second Trimester of Pregnancy

Background and Objectives: Cigarette smoking during pregnancy may adversely affect maternal respiratory function and exercise performance. This study aimed to compare pulmonary function, respiratory muscle strength, functional exercise capacity, and quadriceps muscle oxygenation between smoking and non-smoking women during the second trimester of pregnancy. Materials and Methods: In this prospective cross-sectional study, 110 women in the second trimester of pregnancy (55 smokers and 55 non-smokers) underwent pulmonary function testing, respiratory muscle strength assessment, a six-minute walk test (6MWT), and quadriceps muscle oxygenation measurement using near-infrared spectroscopy (NIRS). Results: Baseline demographic and obstetric characteristics were comparable between the groups (all p > 0.05). Compared with non-smokers, pregnant smokers had significantly lower forced vital capacity (FVC) and forced expiratory volume in one second (FEV1), including % predicted values and z-scores, as well as a lower FEV1/FVC ratio (all p < 0.001). Smokers also had lower maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) (both p < 0.001) and a shorter six-minute walk distance (6MWD) (p < 0.001). In addition, smokers exhibited lower resting and end-exercise quadriceps muscle oxygen saturation (SmO2), together with a greater exercise-induced decline in SmO2 (ΔSmO2) (all p < 0.001). In multivariable regression analysis, pack-years (β = −0.512, p < 0.001) and gestational age (β = −0.217, p = 0.006) were independent predictors of ΔSmO2. Conclusions: Maternal smoking during the second trimester is associated with modest reductions in pulmonary function, respiratory muscle weakness, reduced exercise capacity, and compromised skeletal muscle oxygenation. These findings suggest that the physiological effects of smoking extend beyond the lungs to peripheral muscle oxygen utilization and further emphasize the importance of early smoking cessation during pregnancy.