Analgesia or anesthesia is frequently used for women in labor. A wide range of opioid analgesics with vastly different pharmacokinetics, potencies, and potential side effects can be considered by physicians and midwives for laboring patients requesting pain relief other than a labor epidural. The past 50 years have seen the use of the classic mu opioid agonist morphine and other opioids diminish markedly for several reasons, including availability of epidural anesthetics, side effects, formulary restrictions, and concern for neonatal respiratory depression. Morphine is now primarily used in obstetrics to provide rest and sedation as appropriate for the stressed prodromal stages of a labor without sufficient cervical dilatation. This review discusses the scientific basis for opioid modulation of oxytocin release from the posterior pituitary and the practical implications of this relationship to explain well-known clinical observations of the effect of morphine on prodromal labor.