I bred my mare and she had an ultrasound at 14 days, and the vet said she was open, and had a medium to large follicle. Knowing my mare cycles very regularly, I expected her to be in heat almost immediately after checking as open. That was a week ago, and she still has not come in heat. She is stabled next to the stallion, and it is generally very obvious when she's in heat. I have been watching her very carefully, and she has shown no signs of heat, and her urine is clearer than normal for her, the only other time I've seen it this clear is when she's been pregnant. The stallion has shown no interest in her. I know it's possible for the vet to have been wrong, could she have missed the pregnancy and mistaken a CL for a follicle? Should I follow up with a blood test, get another ultrasound (and if so, when should i do it?), or simply observe her for a few more days? We're now 21 days from the last cover, and 25 days from the first.
Mares develop (and can even ovulate) diestrus follicles, so follicular presence at ~14 days after the previous ovulation really has no significance on way or another. In the same vein, it would not be an indicator that the mare was about to come into estrus ("heat").
Your best bet is to have the mare ultrasounded again, which should be done no later than 26 days after then last cover in order to avoid the possibility of there being twins and the reduction of one leading to the loss of the other after 30 days or so, which could result in the mare not coming back into estrus.
I am aware that follicles don't indicate the mare should come into heat, but I am wondering why the vet would have thought she saw a follicle if the mare is in fact pregnant. What else could she have been seeing? My mare almost always comes back into heat 14-15 days after ovulation, so the fact that she did not (and still has not) is why I wonder if the vet missed something. She is a very good vet, and seemed quite knowledgeable, but she hasn't been a vet for long, and it's possible lack of experience might be the issue here. Unfortunately I can't haul the mare back to the clinic for another week or two. I will certainly get another ultrasound as soon as possible though.
There's no reason that they couldn't have seen a follicle if the mare was pregnant. It's perfectly normal for pregnant mares to develop follicles 16-18 days after they were bred - or not, which is equally normal. Follicles (or the absence of them) at that stage of pregnancy mean nothing.
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