Infant birth is one of the splendid and holy moments of creation but maternal anxiety and pain lead to changes in vital signs and uncontrollable behaviors. At Shahrivar 17 Hospital of Mashhad, Iran, this clinical trial was carried out with 62 primigravidas assigned into control and experimental groups through random allocation. The subjects were healthy primigravidas with full-term pregnancy, single fetus, spontaneous onset of labor, cervical dilation of 3 cm, intact membranes, contractions with 5-10 minutes interval and 20-30 seconds duration. The experimental group received continuous midwifery support by the researcher using Checklist from admission through the second stage. On the contrary, the control group received only the routine care. Their behavioral responses (such at crying, shouting, and scratching) were observed and recorded hour by hour and their physiologic responses (including systolic and diastolic BP, breathing and pulse rate and temperature) were recorded every two hours up to the end of the second stage. The findings revealed that the severity of behavioral responses, at all times, was lower in the experimental groups so that the differences were significant in the first hour (p=0.004), second hour (p=0.002), third hour (p=0.016) and second stage (p=0.002). As for the physiologic responses, the experimental group showed higher mean of diastolic BP in the second (p<0.000l) and fourth hours (p<0.000l); lower mean of systolic BP in second (p<0.000l) and fourth hours (p<0.000l); lower breathing rate in second and fourth hours (p<0.000l) as well as lower temperature mean in second hour (p<0.000l) which were all significant. Findings support the idea that continuous midwifery support in labor can help alleviate behavioral and physiologic responses to labor pain.