Influence of Progesterone Withdrawal on Pregnancy Related Parameters During Post-implantation Early Pregnancy Loss

Introduction 
The dog is a multiparous species, thus normal canine pregnancy is characterized by a number of conceptuses. Integrity of pregnancy is maintained by sensitive interaction of hormones from the hypothalamo-pituitary-gonad axis on one hand and placental factors on the other hand. Insufficient secretion of progesterone by the developing corpora lutea may cause embryonic death prior to implantation occurring clinically unperceived. Death of implanted embryos is followed by so called “resorption” of the complete gestational sac, which may also not be detected by external inspection of the bitch but can be identified by ultrasound. Fetuses dying in progressed pregnancy are usually expelled from the uterus at abortion. Pregnancy can be terminated by withdrawal of progesterone leading to resorption or abortion depending on the stage of gestation at start of treatment. Induction of luteolysis by prostaglandin F2 or analogues alone or in combination with cabergoline during early post implantation pregnancy have shown to induce embryonic death asynchronously in the different conceptuses [9]. In the vast majority of studies on pregnancy termination by blockade of progesterone receptors an antigestagene (aglepristone) has been administered in mid pregnancy or later [6,7]. Nevertheless, clinical use of aglepristone for early post-implantation pregnancy termination has revealed asynchronous embryonic death. Single embryos may even survive and further develop to term if treatment efficiency is not controlled and treatment is not repeated. From these observations it is considered that early post-implantation pregnancy is a specifically stable condition. The aim of this paper is to compile the present knowledge of pregnancy related processes added with some results of a recent study considering both clinical and scientific aspects of early pregnancy loss in the bitch. [...]