Pelvic neurectomy abolishes the fetus-expulsion reflex and induces dystocia in the rat.

To clarify the origin of dystocia in bilaterally pelvic neurectomized (BPN) rats, the uterine activity during the periparturient period, and reflex straining movements in response to vaginal stimulation and cervical extensibility were monitored. The BPN rat displayed "stretching" movements and the regular continuous uterine activity with a similar onset and intensity to that in control rats (intact, sham-operated or unilateral pelvic neurectomized rats) on days 22 or 23 of gestation, but lacked the "straining" movements which precede each delivery of pups in controls so that parturition was prolonged. Dissection of rats revealed that an unborn fetus was retained in the expanded upper portion of the vaginal cavity. Vaginal distension induced contraction of the abdominal muscles and diaphragm and an inhibition of respiratory movements, resulting in an increase in intraabdominal pressure in urethane-anesthetized control rats. This reflex muscular contraction which reflects the straining movements in normal delivery may be called the fetus-expulsion reflex and does not occur in BPN rats. It differs from the fetus-ejection reflex (Ferguson reflex) which initiates oxytocin release. There was no difference in weight and distensibility of the uterine cervix between sham-operated and operated rats on day 22 of pregnancy. The results suggest that the main abnormality of delivery in BPN rats, a prolongation of the process of delivery, may be due to a lack of the fetus-expulsion reflex which plays a physiologic role in removing a fetus from the upper vaginal cavity against the resistance of the pelvic outlet.