This study examined the effects of a yoga program during pregnancy, on maternal comfort, labor pain, and birth outcomes. A randomized trial was conducted using 74-primigravid Thai women who were equally divided into two groups (experimental and control). The yoga program involved six, 1-h sessions at prescribed weeks of gestation. A variety of instruments were used to assess maternal comfort, labor pain and birth outcomes. The experimental group was found to have higher levels of maternal comfort during labor and 2h post-labor, and experienced less subject evaluated labor pain than the control group. In each group, pain increased and maternal comfort decreased as labor progressed. No differences were found, between the groups, regarding pethidine usage, labor augmentation or newborn Apgar scores at 1 and 5 min. The experimental group was found to have a shorter duration of the first stage of labor, as well as the total time of labor.