Final answer:
During pregnancy, the volume of each inspiration decreases due to the upward pressure of the growing uterus on the diaphragm, which reduces lung expansion space and can lead to shortness of breath.
Step-by-step explanation:
The respiratory system parameter that decreases during pregnancy is the volume of each inspiration. During the second half of pregnancy, while respiratory minute volume increases by 50 percent to meet the higher oxygen demands and metabolic rate, the growing uterus presses against the diaphragm. This pressure reduces the space available for the lungs to expand, thus decreasing the volume of air that can be inhaled with each breath, an issue that can lead to dyspnea, or shortness of breath. However, as pregnancy progresses and lightening occurs (when the fetus descends lower in the pelvis), dyspnea may be ameliorated. Furthermore, although respiratory rate may rise, the actual air volume per breath is reduced during pregnancy.