Labor analgesia is an essential health caring procedure for women. However, epidural analgesia cannot be performed on all subjects for different contraindications, such as lower platelet counter, back infection at the puncture site, and fear of epidural injection etc. Therefore, intravenous analgesia is an alternative for such conditions. Given the influence of intravenous administration of drugs on fetus, the drug selection is very important. Remifentanil, a super-short efficacious opioid, can last for 3-4 minutes after injection, which is similar in both maternal and fetal environment. Thus the fetus-associated side effects would be less than other drugs. The investigators hypothesized that remifentanil would be a superior intravenous drug used with patient-controlled technique for labor analgesia.

Inclusion Criteria:

1. Nulliparous women
2. > 18 years and < 45 years
3. Spontaneous labor
4. Analgesia request
5. Epidural puncture contraindications
6. Tendency of bleeding

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