Equine Pregnancy: Physical Interactions Between the Uterus and Conceptus

This report considers the equine intrauterine conceptus from the time of entry from the oviduct as a blastocyst to expulsion as a foal. A series of events involving physical interactions between the uterus and conceptus will be highlighted, and the associated morphologic and physiologic aspects will be considered. Most of the events were discovered and characterized by transrectal ultrasonic imaging and transcervical endoscopic viewing. Certain aspects of the phenomena have been reviewed for embryos,1–3 fetuses,4 and both stages.5,6 The term embryo is well engrained and will be used in reference to the entire early conceptus.5 The terms embryonic vesicle and embryo proper will also be used when needed to emphasize or distinguish between the entire conceptus and the forerunner of the fetus, respectively. The embryo terms will be used only to Day 39 (Day 0=ovulation). From Day 40 to parturition, the terms fetus or fetal stage will be used. The choice of Day 40 as the transition day facilitates categorizing and discussing the phenomena highlighted in this paper. The beginning of umbilical cord formation, completion of replacement of the yolk sac with the allantoic sac, and the beginning of fetal activity (head nods) are on approximately Day 40. Equine theriogenologists and biologists should be deliberate in using the terms embryo and fetus. For example, the terms embryo mobility versus fetal mobility and embryo reduction versus fetal reduction involve distinctly different mechanisms for embryos versus fetuses.