Final answer:
During pregnancy, the respiratory minute volume increases to compensate for the fetus' oxygen demands, causing shortness of breath. As pregnancy progresses, the pelvis becomes more elastic, alleviating dyspnea.
Step-by-step explanation:
During the second half of pregnancy, the respiratory minute volume (volume of gas inhaled or exhaled by the lungs per minute) increases by 50 percent to compensate for the oxygen demands of the fetus and the increased maternal metabolic rate. The growing uterus exerts upward pressure on the diaphragm, decreasing the volume of each inspiration and potentially causing shortness of breath, or dyspnea. During the last several weeks of pregnancy, the pelvis becomes more elastic, and the fetus descends lower in a process called lightening. This typically ameliorates dyspnea.