Lung Volumes . Upward displacement by the uterus causes a 4 cm elevation of the diaphragm, but total lung capacity decreases only slightly because of compensatory increases in the diameters of the chest, as well as flaring of the ribs. These changes are brought about by hormonal effects that loosen ligaments.
From the middle of the second trimester, expiratory reserve volume, residual volume and functional residual volume are progressively decreased, by approximately 20% at term.