The purpose of this study was to compare the physiologic responses to low-impact aerobics using treadmill walking as a control. Ten pregnant women between 21 and 28 weeks of gestation completed 40 minutes of low-impact aerobic dance. The maternal and fetal responses were then compared to 40 minutes of walking at the same heart rate. The aerobics program consisted of a 10-minute warm-up, 20 minutes of high-intensity exercise, and 10 minutes of decreasing intensity. Heart rates were recorded every 5 minutes, and oxygen uptake (VO2) and fetal response (real-time ultrasound) were obtained every 10 minutes. The maternal heart rates were similar during both trials (overall, 133 +/- 6 beat/min). VO2 values during walking were about 4 mL/kg/min greater than during aerobic dance (p < or = 0.003). Minute ventilation (VE) was also greater during walking (28.7 +/- 6.4 versus 24.1 +/- 3.4 L/min, p < or = 0.001). Respiratory exchange ratios and the ventilatory equivalents for oxygen (VE/VO2) were similar for both trials. Aerobic dance caused greater fetal heart rates than walking (p < or = 0.001), differences being as high as 25 beat/min. The fetal rates had returned toward rest within 5 minutes following exercise. Low-impact aerobic dance, compared with walking at similar heart rates, results in a lower maternal metabolic rate and increases the transient stress on the fetus.