The participants: 1800 full-term primiparas will be selected with ASA I-Ⅱgrade, 20 to 35 years old and weighing 55 to 90 kg in the investigator's hospital from may 2018 to December 2018. 1200 primiparas were divided into A and B according to the computer randomized numbers, and 600 parturients without labor analgesia in the same period were selected as group C, with 600 patients in each group. All parturients signed the informed consent form and were approved by the hospital ethics Committee.

1200 primiparas were divided into A and B according to the computer randomized numbers, and 600 parturients without labor analgesia in the same period were selected as group C, with 600 patients in each group. All parturients signed the informed consent form and were approved by the hospital ethics Committee.

Inclusion Criteria:

• ASA I-Ⅱ, aged 22 - 42,
• gestational age ≥ 37, weight 55 - 90 kg,
• labor analgesia is required by full-term primiparas.

Exclusion Criteria:

• serious cardiovascular and cerebrovascular system diseases,
• bradycardia, conduction block
• fetal distress
• contraindications of intraspinal anesthesia.

Female