In order to test whether trained (n = 10) and sedentary (n = 29) pregnant women have different responses to weight-bearing exercise in the second trimester (range 23 to 28 weeks of gestation), subjects walked continuously on a treadmill for 26 minutes: at low intensity for 10 minutes, then an intermediate stage, followed by moderate intensity for 10 minutes. In the trained group, the mean heart rate was lower (p < 0.02) with both low [104.0 +/- 4.5 (SD) versus 117.4 +/- 2.3 bpm] and moderate intensity exercise (128.4 +/- 5.7 versus 145.2 +/- 2.9 bpm) and the associated mean stroke volume was higher (105 +/- 16 versus 84 +/- 17 ml; 103 +/- 15 versus 86 +/- 18 ml, respectively) (p < 0.02). The mean cardiac output, blood pressure and oxygen consumption were the same in the two groups (p > 0.02) at the same absolute workloads. The potential implications of these findings for exercise prescriptions for pregnant women, research evaluation and exercise testing are discussed.