It is in the uterus that developmental characteristics of the conceptus in the horse become especially distinct from those of other domestic species. 

First of all, the equine embryo, its fetal membranes and its circulatory system develop to an extraordinary extent before even the first signs of a functional yolk sac choriovitelline placenta at day 22 (Enders et al., 1993) and allantochorionic placenta at about 40 days after ovulation. 

Second, the endodermal wall of the yolk sac is completed from cells that are unusually broadly distributed around the interior of the blastocyst, and which seem to have unusual functional characteristics (Enders et al., 1993). 

Third, although as in the other species the zona pellucida is shed from the blastocyst, it is replaced in the horse by a glycoprotein capsule of considerable functional importance between approximately days 7 and 21. 

Fourth, the encapsulated conceptus interacts with its environment during two distinct phases; it is extremely mobile until about days 16–17 and then becomes immobilized ‘‘fixed’’ at the site of eventual placentation. 

Fifth, the capsule maintains a spherical form for the conceptus, particularly before fixation. 

Sixth, it is relatively easy to recover the intact conceptuses transcervically from the standing mare at least until about day 35 — a feature important to experimental investigation.

There are numerous structural changes associated with conceptus development during the second and third weeks of pregnancy