Chronic uterine distention and its relation to the end of gestation

Abstract

At the outset, a review of the local physiologic effects of chronicuterine distention under various hormone conditions is given. It isshown that uterine growth resulting from distention takes place inuntreated, ovariectomized rabbits and in rabbits under the influenceof progestin. When estrin is the predominant hormone, the capacityof the distended uterus to grow is appreciably reduced. With these facts as a basis, analysis is made of Hammond’s data onuterine and fetal weights at different stages of gestation in the rabbit.It is shown that a disproportion exists between the growth incrementsof the fetuses and of the uterus in the last third of pregnancy, theformer growing much more rapidly than the latter. It is furthershown that the reason for the increasing disproportion of these growthrates is due largely if not entirely to an increase in the influence ofthe hormone, estrin. Mention is also made of the fact that estrin isthe hormone demonstrated to have the property of imparting rhythmiccontractility to the uterus. As a consequence of the limitation of thecapacity of the uterus to grow, along with the continued increase inthe size of its contents, it is pointed out that the developing rhythmicuterine contractions are rendered increasingly more efficient and forceful.The theory is advanced, therefore, that these physiologic conditionsare the underlying factors which are essential to the onset oflabor, and the commencement of parturition is the result of a gradualand accelerating convergence of these factors, bringing about nutritionalchanges and an appropriate physical orientation of the fetuses.Finally, the common physiologic basis between these conditions favoringthe onset of labor and those which may be responsible for spontaneousabortion is discussed. At the outset, a review of the local physiologic effects of chronicuterine distention under various hormone conditions is given. It isshown that uterine growth resulting from distention takes place inuntreated, ovariectomized rabbits and in rabbits under the influenceof progestin. When estrin is the predominant hormone, the capacityof the distended uterus to grow is appreciably reduced. With these facts as a basis, analysis is made of Hammond’s data onuterine and fetal weights at different stages of gestation in the rabbit.It is shown that a disproportion exists between the growth incrementsof the fetuses and of the uterus in the last third of pregnancy, theformer growing much more rapidly than the latter. It is furthershown that the reason for the increasing disproportion of these growthrates is due largely if not entirely to an increase in the influence ofthe hormone, estrin. Mention is also made of the fact that estrin isthe hormone demonstrated to have the property of imparting rhythmiccontractility to the uterus. As a consequence of the limitation of thecapacity of the uterus to grow, along with the continued increase inthe size of its contents, it is pointed out that the developing rhythmicuterine contractions are rendered increasingly more efficient and forceful.The theory is advanced, therefore, that these physiologic conditionsare the underlying factors which are essential to the onset oflabor, and the commencement of parturition is the result of a gradualand accelerating convergence of these factors, bringing about nutritionalchanges and an appropriate physical orientation of the fetuses.Finally, the common physiologic basis between these conditions favoringthe onset of labor and those which may be responsible for spontaneousabortion is discussed.