Validated target heart rate (THR) zones for exercise prescription for overweight and obese pregnant women have not been developed. The purposes of this study were to determine if heart rate reserve (HRreserve) is best described by aerobic capacity at peak exercise or by aerobic capacity reserve (VO2 reserve) and to develop and validate THR zones for light-intensity exercise (20%-39%VO2 reserve) in sedentary overweight and obese pregnant women. One hundred six women between 16 and 20 weeks gestation with medical clearance performed a progressive treadmill test to volitional fatigue (peak). Data from every 4th subject were used for cross-validation. Two linear regression equations were performed for each subject, then pooled to obtain mean group values (+/- SD): %HRreserve vs. %VO2 peak and %HRreserve vs. %VO2 reserve. THR zones equivalent to 20%-39%VO2 reserve were developed and validated based on the strongest relationship. %HRreserve had a stronger linear relationship with %VO2 reserve (y = 1.046x -7.561; R2 = 0.741) than %VO2 peak (y = 1.259x -28.795; R2 = 0.604). Validated THR ranges for sedentary overweight and obese pregnant women are 102-124 beats.min-1 (20-29 years of age) and 101-120 beats.min-1 (30-39 years of age), representing an exercise intensity of 20%-39%VO2 reserve as recommended by the American College of Sports Medicine for previously sedentary pregnant women. Overweight and obese women who are medically prescreened can exercise during pregnancy within our validated THR zones. The relationship between HR and VO2 remains strong, but the two are not equivalent in this population group.