Mare bred on 6/24. On regumate because she had EED on last breeding. 14 preg u/s shows embryo in tip of right horn. 16 day u/s shows embryo now in mid left horn bumped up against large cyst. At 28 days embryo gone. Tone great. Off regumate. Checked 5 days later not coming in. Still feels pregnant. Still no embryo found. U/S again today, vet says she feels exactly like a 35 day pregnant mare but ultrasound still just shows the same big cyst. He is hesitant to prostin and wants to check again next Tuesday and draw blood for progesteron level. He is very a competent repro vet. Is it possible she is pregnant and the embryo is being hidden by the cyst?
If the mare lost the pregnancy after about 28 days, there is the possibility that the endometrial cups have started forming and eCG is secreted and she is not going to come back into estrus until 90-120 days or so. If this is the case, giving Prostaglandin F2α ("Prostin") will be a waste of time anyway.
If you wait until about day 40-45 and then run an assay for eCG, that will tell you if the endometrial cups are functional. It won't reliably tell you if the mare is pregnant, as the above scenario may be in place (endometrial cups and therefore eCG, but no pregnancy). However, if at that point you find no eCG, you could go ahead and use Prostin with a reasonable degree of security that there is indeed no pregnancy.
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