Another explanation.
Connelly
N R
NR
Dunn
S M
SM
D015360:Analgesia, Epidural; D016362:Analgesia, Obstetrical; D000775:Anesthesia, Spinal; D019162:Anesthetics, Combined / Q000008:administration & dosage; D002045:Bupivacaine / Q000008:administration & dosage*; D002584:Cervix Uteri / Q000187:drug effects*; D005260:Female; D006801:Humans; D007743:Labor, Obstetric; D011247:Pregnancy; D017409:Sufentanil
D019162:Anesthetics, Combined; D017409:Sufentanil; D002045:Bupivacaine