Maternal endocrine status during the first 14 days of pregnancy is similar to that of the nonpregnant mare in diestrus. If the mare is not pregnant, the endometrium releases prostaglandin-F ₂α (PGF ₂α) on approximately days 14 to 15 after ovulation, which causes regression of the corpus luteum and permits the mare to return to estrus. If the mare is pregnant, the corpus luteum does not undergo lysis on days 14 to 15 but persists and continues to secrete progesterone, which is responsible for pregnancy maintenance. The process whereby luteolysis is prevented by the presence of the conceptus is referred to as maternal recognition of pregnancy.

The embryonic vesicle becomes stationary by approximately day 16 after ovulation. This process is referred to as fixation and is apparently largely the result of restriction of the enlarging conceptus at the base of one uterine horn. The process of embryonic vesicle fixation should not be confused with the process of fetal-maternal attachment. Soon after the time of fixation, the cross-sectional shape of the embryonic vesicle typically changes from circular to triangular; this change in shape is identified by transrectal ultrasonographic examination. The change in vesicle shape is the result of thickening (hypertrophy) of the dorsal uterine wall (see Figures 5-30 and 5-31). The encroachment of the dorsal aspect of the endometrium onto the vesicle is thought to play a role in orienting the embryo proper to a ventral 6 o’clock position, opposite the mesometrial attachment (antimesometrial).

When the reproductive tract is under progesterone influence from the primary CL of pregnancy and estrogen influence from the developing conceptus (estrogen is produced by day 12), the uterus begins to develop a characteristic shape with marked tone that becomes readily apparent on palpation per rectum by day 16 to 18 after ovulation (Figure 7-1). Examination via palpation per rectum usually reveals suggestive, but not definitive, evidence of early pregnancy at this point because the conceptus may not yet be readily palpable. The conceptus does not begin attaching to the endometrium until day 40 to 45 of gestation, so the increased tone of early pregnancy is thought to help keep the embryo and developing membranes in close apposition with the endometrium during this time period to maximize nutrient transfer. The advent of ultrasonography has allowed considerable progress in pregnancy diagnostics in the mare because this technique enables detection of embryonic vesicles in the uterus as early as 9 to 10 days after ovulation. Pregnancy can typically be confirmed via palpation per rectum by most examiners at 25 to 35 days of gestation.

The process of fetal-maternal attachment is gradual. The vascularized trophoblast is closely associated with the uterine epithelium by day 25. Beginning interdigitation of trophoblastic microvilli and uterine epithelium has been described as occurring by days 38 to 40. Fetal macrovilli (which become the microcotyledons and are distinct from microvilli) begin appearing by day 45 as rudimentary structures and gradually develop until full placental attachment occurs in the form of well-developed microplacentomes (microcotyledons with maternal microcaruncles) by day 150 (Figure 7-2).

A unique feature of the equine placenta is the development of endometrial cups early in gestation. On about day 25 of gestation, a specialized annular band of the trophoblast undergoes cellular changes to form the chorionic girdle at the junction of the developing allantois and regressing yolk sac. These trophoblastic cells of the chorionic girdle invade the adjacent uterine epithelium on approximately day 38 of gestation, migrating into the underlying lamina propria to develop into the prominent decidua-like cells of the endometrial cups. The endometrial cups are identified as a circular or horseshoe arrangement of pale irregular outgrowths on the luminal surface of the gravid uterine horn (Figure 7-3). They attain maximal size at about day 70 of gestation and then begin to undergo degeneration and eventually are completely sloughed by approximately day 130 of gestation. The endometrial cups secrete a hormone known as equine chorionic gonadotropin (eCG), previously termed pregnant mare serum gonadotropin (PMSG). Equine chorionic gonadotropin is first detectable in maternal blood on days 35 to 42, rapidly increasing in concentration to peak levels on days 55 to 65; then concentrations decline slowly to low or nondetectable levels by days 100 to 150. This hormone is believed to assist in the formation of supplementary corpora lutea and is thought to also be a necessary stimulus for maintenance of the primary CL during approximately days 35 to 120. Progesterone output from the primary CL actually increases during days 35 to 40, before acquisition of significant progesterone production by the supplementary corpora lutea. Equine chorionic gonadotropin is also thought to play an important immunoregulatory role during pregnancy.

Endometrial cup formation is an important consideration in the management of early embryonic loss. If pregnancy loss occurs before the formation of the endometrial cups (days 36 to 40), the mare returns to estrus within a short period of time (less than 1 month). Because only the primary CL is present, a single injection of PGF ₂α promotes CL regression and return to estrus. If pregnancy loss occurs after endometrial cup formation, eCG production continues and the mare may not return to estrus for 3 months, when the endometrial cups degenerate, thereby allowing supplementary corpora lutea to regress. In addition, a mare that loses a conceptus after days 36 to 40 of gestation remains positive for pregnancy according to eCG detection tests (e.g., Equi-Chek, Endocrine Technologies Inc, Newark, Calif.) for up to 3 months although the mare is no longer pregnant, because these tests assay for presence of circulating blood levels of eCG, not pregnancy itself. Of additional importance, repeated PGF₂α injections may cause a mare that has aborted after 40 days of gestation to return to estrus by inducing regression of supplementary corpora lutea, but the estrus is seldom fertile as long as endometrial cups remain. Another aggravating problem when attempting to rebreed a mare that has endometrial cups remaining after abortion, even when the mare has been induced to return to estrus with repeated PGF ₂α injections, is the tendency for follicles to fail to ovulate. Such mares have also been noted to ovulate smaller-than-normal follicles, luteinize follicles that do not ovulate, or form structures similar in appearance to hemorrhagic anovulatory follicles (Figure 7-4).