From the American College of Nurse-Midwives.  Nitrous oxide for labor analgesia.
D016362:Analgesia, Obstetrical / Q000379:methods*; D018712:Analgesics, Non-Narcotic / Q000008:administration & dosage*; D005260:Female; D006801:Humans; D048949:Labor Pain / Q000188:drug therapy*; D007743:Labor, Obstetric / Q000187:drug effects; D009609:Nitrous Oxide / Q000008:administration & dosage*; D010147:Pain Measurement; D011247:Pregnancy; D012956:Societies, Nursing; D014481:United States
D018712:Analgesics, Non-Narcotic; D009609:Nitrous Oxide