We studied the respiratory mechanics, acid-base status, alveolar ventilation and ventilatory response to hypercapnia in pregnant women at the first and third trimesters and postpartum. We found alveolar hyperventilation and hypersensitivity of the respiratory centers to carbon dioxide with no significant differences between the first and third trimesters. After delivery we found a sudden decrease in these parameters. The changes seem to be due to a direct primary (probably hormonal) stimulation on the medulla oblongata and cannot be correlated to the metabolic needs of the fetus and are aimed at favoring placental gas exchanges, especially at the beginning of the pregnancy, when the fetal cardiovascular system is not yet developed.