Several factors influence the extension of anesthetic mixture during spinal anesthesia including anesthetic gravity, body position, drug volume, and drug-delivering velocity. However, the effect of temperature of anesthetic mixture on the cephalad sensory blockade is hitherto unknown. The investigators hypothesized that different temperatures of the anesthetic mixture had different velocity of extension after spinal anesthesia. In addition, previous studies suggest that parturients have relative higher sensitivity to temperature. Herein the investigators proposed that the temperature of anesthetic mixture had more extensive effect on the cephalad sensory blockade with spinal anesthesia for cesarean delivery.

Inclusion Criteria:

1. Nulliparous women
2. > 18 years and < 45 years

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. Alcohol addictive or narcotic dependent patients
5. Subjects with a nonvertex presentation
6. Diagnosed diabetes mellitus and pregnancy-induced hypertension
7. Twin gestation and breech presentation
8. Pregnant fever