BACKGROUND: Excessive stress during pregnancy may cause mental disorders in pregnant women and inhibit fetal growth. Yoga may alleviate stress during pregnancy. AIM: To verify the immediate effects of yoga on stress response during pregnancy. METHODS: One group pre-post test was conducted at a hospital in Japan. We recruited 60 healthy primiparas without complications and asked them to attend yoga classes twice a month and to practice yoga at their homes using DVD 3 times a week from 20 gestational weeks until childbirth. Salivary cortisol and alpha-amylase concentration were measured before and after yoga classes at time 1 (27-32 gestational weeks) and time 2 (34-37 gestational weeks). Subjective mood was assessed using the profile of mood states. Saliva values and mood scores before and after each yoga class were compared using paired t-test and Wilcoxon rank-sum test, respectively. FINDINGS: We analyzed 44 and 35 women at time 1 and time 2, respectively. The mean salivary cortisol concentration declined significantly after each yoga class [time 1: 0.36-0.26mug/dL (p<0.001), time 2: 0.32-0.26mug/dL (p=0.001)]. The mean salivary alpha-amylase concentration also decreased significantly following each class [time 1: 72.2-50.8kU/L (p=0.001), time 2: 70.6-52.7kU/L (p=0.006)]. The scores for negative dimensions of mood (Trait-Anxiety, Depression, Anger-Hostility, Fatigue, and Confusion) decreased significantly. The scores of Vigor for a positive dimension of mood significantly increased. CONCLUSION: This study indicated the immediate stress reduction effects of yoga during pregnancy.