• Maternal blood pressure [ Time Frame: 2 hours ] [ Designated as safety issue: Yes ]
• Maternal heart rate [ Time Frame: 2 hour ] [ Designated as safety issue: Yes ]
• Neonatal Apgar score [ Time Frame: 1 and 5 minutes post delivery ] [ Designated as safety issue: Yes ]
• Mode of delivery (spontaneous vaginal versus instrumental vaginal versus cesarean [ Time Frame: at delivery ] [ Designated as safety issue: Yes ]

Epidural analgesia is widely regarding as the most effective analgesic strategy for labor pain. Modern practice is to utilize dilute local anesthetics as a continuous infusion along with an opioid, e.g., our common "recipe" of 12 ml/hr of 0.0625% bupivacaine with 2 micrograms/ml fentanyl, after the initial dose to maintain patient comfort until delivery. This dose of the infusion often provides adequate comfort without interfering with the mobility of the patient and her ability to effectively push during delivery. However, this low dose epidural infusion strategy often results in recurrence of pain after an initial pain free period.

This breakthrough pain is treated by administering small boluses of analgesics via the epidural catheter. The pain occurring in labor is initially of visceral origin and is mediated by pain fibers originating from the low thoracic and upper lumbar segments of the spinal cord. As labor progresses to the late first phase (also known as transitional stage), pain sensations originating from the distension of the pelvic floor, vagina and perineum adds a somatic component to labor pain. This type of breakthrough pain is often difficult to treat.

Although requests from patients to alleviate late stage breakthrough pain are common, no one knows the most effective strategy for pain management in this stage of labor. This study is designed to compare the efficacy of two treatments for controlling late first stage breakthrough pain during labor with an epidural infusion in place: clonidine-bupivacaine versus fentanyl-bupivacaine.

Women who have labor epidural analgesia in place will be enrolled to be randomized if and when they present with breakthrough pain in the late first stage or second stage of labor (≥ 8 cm dilated). They will receive 8 ml of a solution containing 10 mg bupivacaine and 75 micrograms of either fentanyl (an opioid or "narcotic") or clonidine (an "alpha-2 agonist known to be effective as an epidural analgesic).

Pain relief, labor progress and outcome will be assessed to compare fentanyl versus clonidine.

It is the hypothesis of this study that clonidine added to bupivacaine is a better analgesic than fentanyl added to bupivacaine for breakthrough pain in advanced labor.

• Experimental: Participants randomized to this arm of the study receive 75 micrograms clonidine with the bupivacaine in their epidural when requesting pain relief in advanced labor
• Experimental: Participants randomized to this arm of the study receive 75 micrograms fentanyl with the bupivacaine in their epidural when requesting pain relief in advanced labor

Inclusion Criteria:

• women in labor at term pregnancy
• healthy
• epidural analgesia in place
• breakthrough pain in advanced labor

Exclusion Criteria:

• chronic pain syndrome
• receiving systemic opioids within 4 hours
• receiving chronic antidepressants, clonidine, opioids