Maternal submaximal aerobic fitness (PWC170) was measured before, during, and after 12 weeks of swimming training during the second trimester of pregnancy in 23 sedentary women. For comparison, 11 women maintained their normal activities, but did not swim. Training sessions were 3 times per week for 40 min, producing heart rate (HR) responses of 65%-70% of estimated maximum HR and increases in rectal temperature (n = 8) of approximately 0.4 degrees C. Distance swum per session almost doubled over the training period (581 +/- 177 m to 1110 +/- 263 m). PWC170 of the nonswimming group remained stable over time; that of the swimming group increased significantly by 13.8% after 8 weeks of training (790 +/- 145 to 909 +/- 137 kpm/min; p = 0.026). The results indicate that a significant aerobic training effect can be achieved by light-moderate-intensity swimming during pregnancy in previously sedentary women. Further, all women remained healthy, with no adverse outcomes for mother or baby.