X -- Vets most often miss mares that are 3 - 8 months Pg. May not be a bad overall Vet but one that needs to learn to recognize a 'missing uterus' by palpation. Those are some of the Vets that I have taught how to palpate a mare and not rely on US.
Painted -- When your Vet found Twins, did the Vet 'pinch' one of the Embryos? If so, did the Vet have you put the mare of Regumate for 30 days or give the mare a Progesterone shot? If not, the chances are very high of losing the remaining one. The embryonic fluid from the pinched twin contains a hormone that acts like Prosaglandin and causes the mare to come back in heat, which, of course, causes the loss of the other embryo. That fluid is absorbed by the mare and will frequently cause the mare to shed the other embryo when she comes into heat just like she had been given a shot of Lutalyse. The Progesterone given at the time of the 'twin check' usually lets the mare stay Pg and not come back into heat.