Epidural analgesia in the early stage of labor at cervix < 4.0cm was considered as a risk period for increasing the rate of cesarean delivery. The more recent studies and the investigators' previous data indicated epidural analgesia can be performed as early as the cervical dilation approximately 2.0cm. The investigators hypothesized that different cervix diameter had different rate of cesarean delivery. This trial would investigate the correlation amongst different cervical dilation and the risk of cesarean section in nulliparous women at term.