Study aim: Determination of the effect of 0.2 ml of cerebrospinal fluid aspiration at the end of intrathecal injection of bupivacaine 0.5% on the onset time and sensory level in cesarean section under spinal anesthesia. Design: This clinical trial with a control group with parallel, double-blind, randomized, phase 2-3 groups was performed on 60 patients and randomization block was used for randomization. Settings and conduct: At the end of the drug injection in spinal anesthesia, in 30pregnant patients in group A (aspiration group), 0.2 ml of CSF is re-aspirated and re-injected into the subarachnoid space and in 30 pregnant patients in group B (without aspiration), without CSF aspiration, the spinal needle is removed and the patient is placed in a supine position.Then the time to reach the maximum level of sensory block (using pinprick), the quality of sensory and motor block (using VAS and bromage score), blood pressure and heart rate at specific times (by non-invasive monitoring of Saadat Model 162 made in Iran) are measured and recorded in a questionnaire. Participants/Inclusion and exclusion criteria: Inclusion criteria: Conscious consent to participate in the study; 18 to 46 years old; Term pregnancy (37 weeks and more); Candidate for cesarean section under spinal anesthesia; Exclusion criteria: Cancel the continuation of the plan; ‌ Heart and lung diseases, hypertension, diabetes, arrhythmia and heart failure; Multiple pregnancies; Eclampsia and preeclampsia; Contraindications of spinal anesthesia; Emergency C/S Intervention groups: Patients after spinal anesthesia are randomly assigned to one of the two groups of cerebrospinal fluid aspiration ( Intervension group) and without cerebrospinal fluid aspiration (Control group). Main outcome variables: Time to reach the maximum sensory block and quality of sensory-motor block