I can't fathom not doing internal ultrasounds. If I had to I would haul them to the next state just for the peace of mind of "knowing for sure".
Yes, that does worry me, for the same reasons you state. My regular vet is a very big guy, no way he could/would be able to do an internal US. If there was a vet within a reasonable distance, then I would consider it, but right now, no.I don't have anyone in this area that has small enough hands to do internal ultrasounds for me, and I don't want to stress a possibly pregnant mare by hauling her to someone who can.
I used to have an awesome vet, he had small hands and was great with the mares, and he was really good at reading an ultrasound, or being accurate at palpating. Since he moved though, I haven't dared try any other vets, as the ones I know are just too big, and don't seem comfortable doing so. I've never had a vet with an internal probe so long that they didn't have to put their hand in the mare as well.
Does anyone else worry about the risks to the mares of palpating and ultrasounding (when a hand has to be inserted as well?)?