Which of the following Lung volume remains unchanged during Pregnancy?

Functional residual capacity (FRC) decreases by approximately 20 to 30 percent or 400 to 700 mL during pregnancy. This capacity is composed of expiratory reserve volume which decreases 15 to 20 percent or 200 to 300 mL and residual volume which decreases 20 to 125 percent or 200 to 400 mL. FRC and residual volume decline due to diaphragm elevation, and significant reductions are observed by the sixth monthWith a progressive decline across pregnancy. Inspiratory capacity, the maximum volume that can be inhaled from FRC, increases by 5 to 10 percent or 200 to 250 mL during pregnancy. Total lung capacity: the combination of FRC and inspiratory capacity is unchanged or decreases by less than 5 percent at term.The respiratory rate is essentially unchanged, but tidal volume and resting minute ventilation increase significantly as pregnancy advances.Reference: William&;s Obstetrics; 24th edition; Chapter 4; Maternal Physiology