Why not use naloxone?
D000700:Analgesics / Q000037:antagonists & inhibitors*; D005260:Female; D006801:Humans; D007748:Labor Stage, Second; D008431:Maternal-Fetal Exchange; D009270:Naloxone / Q000008:administration & dosage*; D011247:Pregnancy
D000700:Analgesics; D009270:Naloxone