 Our hypothesis is that obese women require less intrathecal hyperbaric bupivacaine than do normal weight women to achieve satisfactory surgical anesthesia for elective Cesarean section.Two groups of patients will be studied separately. Patients with BMI greater than or equal to 30 will be included in the obese group, also denominated study group. Patients with BMI less than 25 will be included in the normal weight group, also denominated control group. BMI will be calculated based on the patients pre-pregnancy weight. For each group, the up-down sequential allocation method based on the Narayana rule will be used to find the minimum effective dose of intrathecal hyperbaric bupivacaine 0.75 associated with opioids necessary for satisfactory outcome in 95 of the pregnant women undergoing cesarean section, i.e. ED95. 