Effect of Morphine on Labor. ∗

Abstract

The occurrence of respiratory depression and failure at birth following the administration of morphine for relief of pain during labor has prompted numerous clinical investigations. Two chief questions arise regarding the mechanism by which administration of morphine to the mother may result in injury of the fetus. There is involved firstly, the effect of morphine upon the fetus directly; and secondly, the effect of morphine upon the labor mechanism. Evaluation of these factors in clinical studies has been difficult because of the complications present at the time of delivery which may result in injury to the child such as anesthesia, anoxemia and∗ mechanical trauma. Accordingly, the present experiments have been undertaken in an attempt to control conditions of observation so that it could be determined whether or not the effect of morphine upon the fetus is of greater consequence than the effect of the narcotic upon the course of labor.

The effect of morphine on the fetus was first studied by direct observation of full-term rabbit fetuses within the unopened uterus according to a method previously described. 1 The change in rate of fetal respiratory movements afforded a sensitive indicator of the depressant effect of the narcotics. Results showed that rhythmical breathing of the fetuses persisted despite administration to the mother of more than 15 times the analgesic dose of morphine. 2

Additional evidence regarding the state of narcosis of the fetus at various intervals following injection of morphine was obtained in a second series of animals in which the rabbits were not prepared for observation of intrauterine respiration. A dosage of 13 mg per kg of morphine was given by a single intravenous injection in rabbits about the time of the onset of labor, i. e., at 31 or 32 days.