Intra-uterine pressure changes during pregnancy and parturition in rabbits.

Abstract

SUMMARY Small balloons attached to polyethylene tubing were inserted into either end of one uterine horn in rabbits on about the 22nd day of pregnancy. The tubing was threaded subcutaneously and brought out through an incision at the back of the neck; recordings of intra-uterine pressure changes were made up to and including parturition. There were no pressure changes which could be ascribed to contraction of the myometrium until an average of 31 hr. before delivery of the first foetus but there was considerable variation. Pressure waves recorded from the two balloons were often different in frequency and amplitude and were seldom synchronous. In none of the rabbits did one end of the horn develop 'parturient pressure' before the other and, in general, pressure at the ovarian end of the uterus was higher than at the cervical end. The rabbits could be divided into two groups: (I) those in which the onset of delivery was abrupt and preceding pressure waves were of low amplitude; (II) those in which the pressure waves increased gradually in intensity over several hours before delivery. Both the duration of delivery and the mortality rate of the foetuses were significantly greater in group II than in group I. The results in group I are consistent with the concept that in the normal rabbit parturition is initiated by a release of oxytocin. It is suggested that in group II oxytocin release failed to occur and that this failure was due to 'emotional stress' and possibly circulating adrenaline.