In this study, the investigators aim to compare the duration of analgesia of intrathecal morphine vs. hydromorphone for analgesia after cesarean delivery. Secondarily, the investigators will compare the side effects of each drug, including nausea and pruritus. To achieve the goals of this study, it is important to study equipotent doses of these medications. Previous work by the investigators of this study found that the effective dose for postoperative analgesia in 90% of patients (ED90) is 75 micrograms for intrathecal hydromorphone and 150 micrograms for intrathecal morphine. However, it is not known if these two equipotent medication doses provide a similar duration of analgesia.