[Purpose] We examined cardiorespiratory and metabolic changes across the 1st (G1), 2nd (G2) and 3rd (G3) trimesters in pregnant women. [Subjects and Methods] Forty-two healthy, active, non-smoking, pregnant women participated in this study. They were divided into G1, G2 and G3 groups depending on their mean gestational ages at the time of testing which were 10.5 ±2.9, 19.2 ±3.4, and 33.3 ±2.4 weeks of gestation, respectively. Cardio-respiratory and metabolic variables, VO2 (oxygen consumption), VCO2 (carbon dioxide production), and VE (minute ventilation), were measured using indirect calorimetry (IC, gas analyser) to estimate ventilatory equivalents of oxygen (VE/VO2) and carbon dioxide (VE/VCO2), RER (respiratory exchange ratio) and REE (resting energy expenditure). [Results] Women in the late pregnancy period had higher resting VCO2 and RER, whereas the VE/VCO2 ratio was significantly lower than in G1 and in G2. Even though the values of VO2 and REE increased throughout the course of pregnancy, no significant differences were found. [Conclusion] In pregnant women, resting cardiorespiratory and metabolic variables continuously changed throughout the 3 trimesters. Changes in VE/ VCO2 and RER indicate shifting metabolic energy substrates. In addition, changes in cardiorespiratory variables, in parallel with gas exchange, indicate a better gas exchange process. © 2014 The Society of Physical Therapy Science.