Myometrial activity and expulsion of fetuses

Abstract

Chronic implantation of extracellular electrodes on successive uterine segments of pregnant miniature sows and marking of their fetuses were performed in order to relate characteristics of uterine electrical activity during delivery with the sequence of expulsion of piglets.

The electromyographs during parturition were characterized by synchronic and well propagated phases of activity that could first appear at any of the four electrodes; however, initiation occurred most frequently at the electrodes on the two extreme ends of a horn. Analysis of the relative occurrence of tubocervically and cervicotubally propagated activity phases indicated that the latter were present, and even could dominate, before and during expulsion of the piglets. The mean duration of the activity phases decreased and their mean propagation-time along the horn increased after emptying of the horn. However, before as well as after completion of delivery, a wide range in propagation-time of even two successive activity phases was measured.

It is concluded that a decrease in uterine volume is responsible for the rather abrupt changes of the characteristics of myometrial activity upon delivery of the piglets from a horn. Possible explanations for the wide range of the propagation-times are discussed and it is suggested that the cervicotubally directed contractions prevents untimely interruption of the fetal-placental-maternal connections of the piglets that are not yet due to be expelled.