Accuracy of Borg's ratings of perceived exertion in the prediction of heart rates during pregnancy.

When using Borg's 6-20 scale during pregnancy, ratings of perceived exertion (RPE) did not significantly correlate with exercise heart rates (HR) (P greater than 0.05). The HR predicted from RPE significantly (P less than 0.05) underestimated the exercise HR in the second trimester during walking (Group 1: mean difference 16 beats min-1, n = 11), aerobics classes (Group 4: mean 15 beats min-1, n = 48) and circuit training (Group 3: mean 18 beats min-1, n = 24); and in the third trimester during cycling (Group 2: mean 16 beats min-1, n = 12) and aerobics classes (Group 5: mean 11 beats min-1, n = 29). Maximal individual HR under-estimations were large for each physical activity during pregnancy, with values up to 54 beats min-1. Consequently, exercise intensity should not be monitored solely with RPE during pregnancy.