Respiratory disorders during pregnancy are connected with its physiology. About half of pregnant women suffer from dyspnea on exertion and some 20 % also from dyspnea at rest. Symptoms may intensify in obese patients. Smoking and respiratory disorders influence the well-being of the fetus. This study evaluates respiratory function in pregnant women as assessed by spirometry. The tests were carried out in 54 pregnant women in the 2nd and 3rd trimester. We found reduced values of vital capacity and expiratory reserve volume in all women, which suggests the existence a restrictive respiratory disorder in physiological pregnancy. Smoking seems to cause obstructive disorders; in smoking patients there was a reduction of the Tiffenau ratio. Participation in birth classes had a positive influence on inspiratory capacity. High BMI before pregnancy, excessive weight gain during pregnancy, or age of becoming pregnant did not appreciably influence spirometry results.