Most ultra sound equipment and vets hands would perforate the uterus or the vagina of a miniature or pony mare. My vet would not even touch my miniatures mares and was hesitant to do my shetland mares. He did ultra sound my shetland mares, but it was a tight fit. He also borrowed miniature speculum equipment from another vet and culture my miniature mares.
This is not uncommon for vets - if you could find a small female vet with small hands, they can try to palpate a miniature mare.
If you want to ultra sound a miniature mare contact Joyce L at
[email protected], she has ultra sound machines that she says works on miniature mares.