 Ropivacaine is one of commonly used anesthetics for spinal anesthesia. Usually L2-3 or L3-4 intervertebral space is chosen for spinal anesthesia. The efficacy of ropivacaine injected into subarachnoid space depends on the given dose and the chosen intervertebral space. Appropriate dose could satisfy the requirement of operation and reduce incidence of the adverse reaction. But it is not certain about the minimum effective dose of ropivacaine in cesarean section through the two intervertebral spaces, respectively. This study is being conducted to find the minimum effective doses for L2-3 and L3-4 spinal anesthesia in cesarean section. 