When we were doing scans of our mares, our vet was able to do a measurement of the ... femur? ... I think. If she could get a good angle. Then, based on that measurement, she was able to guesstimate how far along the mare was. We had one mare that the measurements (3 scans) never matched my breeding dates. She foaled based on my breeding dates - but a really TINY baby. She made up for it though and matured almost as tall as her sire. She was a first foal for that mare.
As to the colors - all 3 of the horses are black based w/ two carrying bay. That means that if the foal isn't buckskin or pali, w/o spots that it could still be by the other stallion...
But time will tell. I'm crossing my fingers you get your buckskin or pali baby from your pair!!
I know now that I hate it when there is any ? as to the sire of a foal. Was quite surprised to find the black and white colt 2 weeks ago. Our 2 yr old stallion was busy - and I don't have any notes of another fence break out during the time frame would have needed to be? So - thru our fence? OUCH... Also now makes me wonder about our chestnut tobi filly - will be having her parentage tested before we do papers - may have to do another amended stallion report. GAK. May turn out that our 2 yr old black tobi stallion is the sire of the chestnut tobi filly as well!