While neuraxial analgesia (epidural, spinal) provides the best pain relief with the fewest side affects, getting one of these done quickly in the emergency department is unlikely; and while a local pudendal or paracervical block is always an option, unless one has done several emergent deliveries using this technique or has time to review Roberts and Hedges, one will probably avoid this as well. This leaves systemic IV analgesics as probably the best option for pain relief during emergency department delivery. Of these, which is best?
Here are a few reasonable options ...
Fentanyl or Remifentanil - rapid onset and offset and hence decreased risk of neonatal respiratory depression
Nalbuphine and other opioids with mixed agonist-antagonist properties - advantage here is a dose ceiling effect with regard to respiratory depression, however psychotomimetic side effects can often occur.
Source
Grant, G. MD. "Pharmacologic management of pain during labor and delivery" Up to Date. 3 June 2008.