The recommended position of the mother under caesarean section after spinal anesthesia is 15 degrees left. However, recent research has challenged the basic principles and practicality of the left-turn 15 degree position. Higuchi et al used nuclear magnetic imaging to directly prove that in the supine position, the position of the full-left 30 degrees of the full-term pregnancy of the full-term pregnancy relieved the inferior vena cava compression and the left-angle of 15 degrees did not. Therefore, there are more and more controversies about the choice of cesarean section position. This experiment aims to explore effects of different positions (15 degrees left, 30 degrees left and supine) for elective cesarean section on lumbar anesthesia for fetal acid-base balance and maternal Hemodynamic.

When the pregnant woman in the third trimester is in the supine position, the enlarged uterus may oppress the inferior vena cava, thereby reducing the amount of blood and heart output of the pregnant woman. This affects the mother mainly, such as dizziness, nausea and vomiting, chills, and severe irritability，difficulty breathing or even cardiac arrest, the impact on the fetus is mainly the reduction of blood flow in the placenta, affecting the blood exchange of the uterus placenta, causing neonatal respiratory distress, acidosis and even death. The recommended position of the mother under caesarean section after spinal anesthesia is 15 degrees left. However, recent research has challenged the basic principles and practicality of the left-turn 15 degree position. Higuchi et al used nuclear magnetic imaging to directly prove that in the supine position, the position of the full-left 30 degrees of the full-term pregnancy of the full-term pregnancy relieved the inferior vena cava compression and the left-angle of 15 degrees did not. Therefore, there are more and more controversies about the choice of cesarean section position. This experiment aims to explore effects of different positions (15 degrees left, 30 degrees left and supine) for elective cesarean section on lumbar anesthesia for fetal acid-base balance and maternal Hemodynamic.

Inclusion Criteria:

• Singleton pregnancy at term via elective cesarean section
• Height from 150 cm to 180 cm
• American Society of Anesthesiologists (ASA) grade from I to II grade
• BMI（Body Mass Index，mearsured weight divided by height squared）less than 35 kg/m2.

Exclusion Criteria:

• Transverse lie
• Fetal macrosomia
• Uterine abnormalities (e.g., large fibroids, bicornuate uterus)
• Polyhydramnios
• Ruptured membranes
• Oligohydramnios
• Intrauterine growth restriction
• Gestational or nongestational hypertension, diabetes, eclampsia
• Hypertensive disorder or any condition associated with autonomic neuropathy (e.g., diabetes mellitus for more than 10 yr), with renal failure
• Have contraindications for spinal anesthesia（Such as low back infection, spinal deformity, etc.）
• Participants refused to sign informed consent.

IPD will be available when this trial is finished and the article have been published