Between 35 and 40 days the embryo attaches "endometrial cups" to the uterous. Once those cups attach, the mare thinks she's pregnant. So if the mare looses the pregnancy after the cups have formed, it'll be approximately 4ish months before she cleans up everything and starts to cycle again. Its not a hormone, rather this structure. That is very different from a false pregnancy, which doesn't happen in horses. Each part of the process; staying out of heat and forming accessory CL's, bagging up, and "labor", are individual symptoms of seperate problems.
What I am referring to is the hormone prostaglatin and it's role here. The following is part of an article about false pregnancy (pseudo pregnant mares, prolonged luteal activity, if you prefer a different term) in horses. The entire article was huge so only copied this part here.
False Pregnancy
Now that we have an understanding of the role of prostaglandin in the reproductive process, it is time to take an in-depth look at what occurs or doesn't occur that results in a false pregnancy. This time our expert on the subject is Patrick J. Meyers, DVM, MS, Dipl. ACT, University of Guelph in Ontario. He prefers using the term prolonged luteal activity rather than false pregnancy.
"To understand this condition," he writes, "the following points must be considered. Luteal tissue can originate from anovulatory (not ovulating) as well as ovulatory follicles. The corpus luteum resulting from the primary ovulation during the follicular phase of the estrous cycle is present during diestrus (not in heat), pregnancy, and embryonic loss. New or accessory corpus luteums develop while under the influence of a normal (diestrus and pregnancy) or an abnormal (prolonged luteal activity) progestational state. These unique physiologic intricacies of the mare make understanding the term prolonged luteal activity difficult."
Meyers further explains that his rather long statement means that continued function of the corpus luteum to produce progesterone even though pregnancy has not occurred or has been terminated, is brought about by failure of the uterine luteolytic mechanism.
In the absence of an embryo, it is the endometrium of the uterus in the normal horse that produces the prostaglandin that becomes the messenger that informs the mare's reproductive system that something has gone awry, and it is time to start over.
The prostaglandin normally travels from its production site in the uterus to the corpus luteum and presents its message. When the message is received and heeded, the production of progesterone ceases, and the mare returns to estrus.
Let us interject here a word about what happens when a viable embryo exists in the uterus. The embryo attempts to ensure its preservation by traversing and contacting a large portion of the surface of the endometrium during the mobility phase (Days 11 to 15 post-ovulation). By so doing, it prevents the release of prostaglandin, so the corpus luteum continues to function.
"This physiologic phenomenon," Meyers explains, "may be referred to as the maternal recognition of a pregnancy. Loss of an embryo after maternal recognition of a pregnancy causes a prolongation of luteal activity because embryonic secretory products or remnants may remain in contact with the endometrium for several days after embryonic death. When early pregnancy loss could not be easily established before the widespread use of ultrasound, mares may have been diagnosed as 'pseudo-pregnant' if there appeared to be an extended period of anestrus during the ovulatory season. Any severe uterine anomalies or pathology that eliminates or upsets the uterine luteolytic mechanism may also result in prolonged luteal activity through uteropathic causes."
That being said, Meyers goes to what he considers to be the heart of the problem in a number of cases of false pregnancies--undetected ovulations during the diestrous (out of heat) period.
With ultrasound monitoring of ovaries, he tells us, "it was determined that previously undetected diestrous ovulations were responsible for prolonged luteal activity."
The occurrence of these ovulations toward the end of diestrus, he explains, probably created a corpus luteum that was too immature to respond to the prostaglandin sent its way from the uterus.
Diagnosis And Treatment
"If a mare experiences long periods of anestrus combined with irregular estrous cycles during the ovulatory season without being mated," Meyers explains, "diestrous ovulations may be suspected. Careful and constant monitoring of follicular and luteal dynamics through the use of ultrasound is the only way that diestrous ovulations will be detected. Monitoring must be performed every 24 to 48 hours.
"The results of monitoring luteal progression and dynamics through ultrasound and daily plasma progesterone determination are valuable information. These two techniques assist the practitioner in determining the optimal time for the assignment of prostaglandin treatment to induce a mare into a fertile estrus on the subsequent estrus."
Administration of the correct amount of prostaglandin, Meyers says, will "suppress follicular activity and may allow a more orderly 'restart' of the endocrinologic clock."
He concludes his discourse on diagnosis and treatment protocol this way:
"Again, ultrasound is an invaluable tool in the diagnosis of cases of prolonged luteal activity and its causation. Endometrial biopsy may be extremely helpful in the diagnosis of prolonged luteal activity associated with repeated bouts of early embryonic mortality. An endometrial biopsy is also required for ruling out the possibility of uteropathic persistence of the corpus luteum (i.e., endometrial underdevelopment, chronic low-grade endometritis). If prolonged luteal activity can be ascribed to idiopathic (of unknown causes) persistence or early embryonic death, prostaglandin should be administered to induce a return to estrus."
The role of ultrasound in the diagnostic procedure is highly important because rectal palpation of the corpus luteum often is ineffective.
In the textbook, Equine Reproduction, researchers Elaine M. Carnevale, DVM, MS; Angus O. McKinnon, BVSc, MSc, Dipl. ABVP, Dipl. ACT; Edward L. Squires, PhD; and James Voss, DVM, MS, one of the book's editors, collaborated on a chapter that discusses the use of ultrasound. They noted the following information:
"Some of the reasons for ultrasonographic evaluation of corpa lutea are to 1) detect ovulation; 2) evaluate corpus luteum formation; 3) determine size and characteristics of the corpus luteum; 4) determine if failure of a mare to display estrus is caused by prolonged maintenance of a corpus luteum or absence of a corpus luteum and follicular activity; 5) distinguish between anovulatory hemorrhagic follicles, luteinized unruptured follicles, and corpus luteum, and 6) determine if a mare has ovulated more than one follicle."
The four then zeroed in on the use of ultrasound to determine if a pseudopregnancy (false pregnancy) is being dealt with. They point out that, with ultrasound, prolonged maintenance of the corpus luteum, resulting in false pregnancy, can be differentiated from an anovulator or anestrous condition. The corpus luteum is first visible on the day of ovulation (Day 0) as a strongly echogenic (giving rise to echoes of ultrasound waves), circumscribed mass of tissue.
The echogenicity, they reported, gradually decreases throughout diestrus. However, just before regression of the corpus luteum, echogenicity increases.
The researchers explained it this way:
"Initially, the corpus luteum is highly echogenic on the day of ovulation. At this time, it is easiest to identify. The echogenicity decreases over the first six days of diestrus, remains at a minimum level for several days during the middle of diestrus, then increases over Days 12 to 16. An increase in brightness of the corpus luteum during the time of corpus luteum regression was also observed. The ultrasonographic changes are apparently indicative of changes in luteal hemodynamics and may be indicative of changes in patterns of blood flow within the corpus luteum as well as changes in tissue density.
"With experience, the practitioner can become accurate at using ultrasonography to confirm ovulation and detect the presence of a corpus luteum. Ultrasonography can also be used to diagnose pseudopregnant mares. A persistent corpus luteum and absence of an embryonic vesicle are evidence of a pseudopregnancy. Once these mares are identified, prostaglandins can be safely given to induce estrus."
Thus, we can conclude, the condition of false pregnancy, though simple on the surface, is a bit more complicated when given an in-depth examination. The good news for the horse owner is that the problem can be diagnosed with ultrasound and normally can be treated effectively with administration of prostaglandins.
The take home message for the horse owner is to make use of the technology that is available. Most major breeding farms today have veterinarians on staff or on call who routinely monitor the pregnancy progress of the broodmare band. A number of smaller breeders, however, put stallion to mare, check them through teasing to determine whether they have come into heat again, and if they do not show signs of estrus, declare the mare to be pregnant. As has been learned from the experts quoted above, this is not always the case.
The mare's reproductive system might have failed to receive the signal that no pregnancy exists and does not return to estrus. The result can be a wasted year. An ultrasound examination can prevent that from occurring, and can be a valuable tool in monitoring fetal development.
