I have a maiden 13 yo Dutch mare that was bred 3 times last year by someone else and failed to conceive (there were a couple of factors going on though...not sure if it was the vet/semen/mare/bad luck). She arrived at my farm in January and I was going to let her stay open this year, but decided to give it one chance. We bred her in the beginning of July with fresh cooled semen. She double ovulated, and much to my surprise, twinned. One twin was pinched at 15 days and she was given banamine and started on regumate. She had one CL at that time but it appeared to be regressing. We checked her around 25 days and the embryo looks good. However, she had a 50 mm follicle and no CL's. So, she will be on regumate probably until 120 days or once the placenta starts producing progesterone. I realize there's a chance that the 50 mm follicle could ovulate, and a CL would result. What's going on here? There's no maternal recognition of pregnancy, but what causes this? Will this continue to happen yearly or could she be totally normal next year? The mare has a IIa biopsy, just a few small cysts, and I did follow her cycles this year just to track her and she cycled normally, didn't pool fluid post breeding....nothing unusual. What gives?
Have you run a blood progesterone assay? Without those results you're not going to get a true picture.
The presence (or absence) of a follicle on the ovary prior to 35 days really means nothing. The CL regressing at about 15 days is not unusual, in fact it is normal, but by about 30-35 days will have undergone luteal resurgence and will be back to secreting progesterone full blast. It should be noted that during that time frame - 15-35 days, the CL - regressing or not - will still be secreting progesterone. HOWEVER, recent research has shown that the use of Regumate can in fact suppress endogenous progesterone production.
By about 36 days, the endometrial cups will have formed, and will be secreting the hormone eCG, which encourages the production of secondary follicles which either ovulate or luteinize and become secondary CLs, which will continue to secrete progesterone until around day 120 of pregnancy.
Nobody knows what triggers maternal recognition in the equine, only that it occurs around day 12-14 and without it, the mare will return to estrus and the pregnancy will be lost through the cervix when it relaxes due to that return to estrus (no such thing as "absorption"!).
My guess is that there was functional luteal tissue present that was not observed on the ultrasound. If you want to know for sure, have a blood-progesterone assay done (or rather, ideally several, as the levels can vary through the day or from day to day) - it's probably cheaper than keeping her on Regumate for 120 days, and besides, you'll have an idea what to expect next pregnancy (the Regumate will not assay as progesterone).
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