In brief:The authors measured metabolic and circulatory responses to a monthly ten-minute submaximal treadmill run at 6 mph in a 32-year-old pregnant runner. Ventilation, oxygen uptake, heart rate, and respiratory exchange ratio during steady-state running all increased substantially as the pregnancy progressed. The increase in oxygen uptake was directly proportional to the weight gained, but the heart rate, ventilation, and respiratory exchange ratio were not. The authors concluded that running becomes more stressful as pregnancy proceeds, and they recommend progressively decreasing running speed.