Different medications can be used as analgesics in labor epidurals. Bupivacaine is one of the most commonly used drugs for that purpose. The efficacy of a certain medication injected epidurally depends on the dose that is given. A certain dose can be administered in different concentrations, which will consequently mean different volumes. Our hypothesis is that for each concentration of a certain drug, there has to be a minimum effective volume that will be associated with the best possible performance of the drug. This study is being conducted to find the minimum volume of bupivacaine (a local anesthetic) that produces successful analgesia in 95% of patients in labor.
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Inclusion Criteria: * ASA I or II * Full term (more than 37 weeks gestation) * Singleton pregnancy, vertex presentation * Regular painful contractions occurring at least every 5 minutes * Cervical dilatation \< 5 cm Exclusion Criteria: * Any contraindication to epidural anesthesia * Accidental dural puncture * Allergy or hypersensitivity to bupivacaine * Women who have received opioids or sedative medications within the last 4 hours