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.

Inclusion Criteria:

1. Nulliparous women
2. > 18 years and < 45 years
3. Spontaneous labor
4. Analgesia request

Exclusion Criteria:

1. Allergy to opioids, a history of the use of centrally-acting drugs of any sort, chronic pain and psychiatric diseases records
2. Participants younger than 18 years or older than 45 years
3. Those who were not willing to or could not finish the whole study at any time
4. Using or used in the past 14 days of the monoamine oxidase inhibitors
5. Alcohol addictive or narcotic dependent patients were excluded for their influence on the analgesic efficacy of the epidural analgesics;
6. Subjects with a nonvertex presentation or scheduled induction of labor
7. Cervical dilation was 5.0cm or greater before performing epidural puncture and catheterization.
8. Diagnosed diabetes mellitus and pregnancy-induced hypertension
9. Twin gestation and breech presentation.