Endometrial Cups
By Patrick M. McCue, DVM, PhD,
Diplomate American College of Theriogenologists

Events of reproduction in the mare are fairly similar to that of other domestic animal species. Typically one dominant follicle develops and produces estrogen that causes the mare to come into estrus or heat.  The dominant follicle ovulates toward the end of estrus and a corpus luteum forms from the site of the former follicle.  The corpus luteum is responsible for producing progesterone that takes the mare out of heat, closes the cervix and is critical for maintenance of pregnancy.

A unique feature of equine reproduction is the formation of structures called endometrial cups during pregnancy.  Endometrial cups form from placental cells that invade into the uterine lining (endometrium) of the mare beginning at approximately 35 days of pregnancy. It is important to understand that the invading cells are from the developing fetus and not from the mare.  The invading cells form a series of discrete masses in a ring pattern at the base of one uterine horn of the mare.  Endometrial cups produce large quantities of a hormone called equine chorionic gonadotropin (eCG) beginning at the onset of cup formation and continuing until the cups disappear at around day 120 to 150 of pregnancy.

An understanding of the physiology of endometrial cups can help a horse owner or breeding manager appreciate management decisions or potential reproductive problems in a pregnant mare.  eCG produced by the endometrial cups stimulates the ovaries of the mare to form additional corpora lutea, which in turn produce more progesterone to support the pregnancy in the first trimester.  Endometrial cups have been considered by many to be a ‘back-up plan’ to help mares maintain pregnancy, in addition to the primary corpus luteum that forms after the initial ovulation.  Another important feature to note is that once formed, endometrial cups will virtually always be persist for 3 to 4 months, whether or not the mare stays pregnant.

Measurement of blood levels of eCG were historically used as a method of pregnancy detection in the mare. Two major problems exist with using eCG levels for pregnancy diagnosis.  First, a false negative diagnosis (i.e. no eCG detected in a mare that is truly pregnant) can occur if a blood sample is collected prior to day 35 before endometrial cups form or after day 120 of pregnancy after regression of the cups.  Second, a false positive pregnancy diagnosis (i.e. elevated eCG detected in a mare that is truly not pregnant) can be made if a blood sample is collected from a mare that lost her pregnancy after endometrial cup formation.  Therefore, detection of elevated eCG in the blood of a mare will only confirm that endometrial cups are present and do not indicate true pregnancy status or fetal health.

It may be important to measure eCG levels in a mare that has lost her pregnancy at approximately day 35 or slightly after.  If levels of eCG are low, indicating that endometrial cup formation did not occur, the mare can be administered prostaglandins and she should return to estrus and can be rebred.  However, if eCG levels are elevated, indicating that endometrial cups are present, the mare will not usually cycle back and she is generally lost for the season. 

Abnormal persistence of endometrial cups has been described in mares with an early termination of pregnancy (i.e. abortion) and mares that carried a normal pregnancy to term.  Persistent endometrial cups have been associated with failure to show behavioral estrus, complete ovarian inactivity, sporadic follicular growth and luteinization of partly developed follicles.  Diagnosis of persistent endometrial cups is based on observation of endometrial cups on transrectal ultrasonography, videoendoscopic evaluation of the uterine lumen, and measurement of circulating levels of equine chorionic gonadotropin (eCG).  Persistent endometrial cups have been documented to last for 6 to 30 months.

The photograph below was taken during an endoscopic examination of a mare that had lost her pregnancy between day 45 and 60.  Endometrial cups can clearly be observed as the slightly raised white plaques adhered to the wall of the uterus.  As stated previously, the presence of endometrial cups will prevent the mare from returning to estrus and being rebred.  As a consequence, this mare was left open for the remainder of the breeding season.

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