The vet yesterday on the phone told me that US after the beginning stages would only show blobs. I asked on equine repro and this was my answer:
The problem with any potential pregnancy detection at this stage (from as early as 65 through to about 150 days) is that the weigh of the pregnancy can cause the fetus itself to drop over the pelvic brim into the abdominal cavity, and out of reach of the ultrasonographer. If it wasn't for that, it shouldn't be a problem for an experienced ultrasonographer to identify a pregnancy or possibly two pregnancies, as there should be a very visible heart beat, as well as other easily identifiable structures (head, thorax showing ribs etc.) - but, as I say, the issue at this stage is actually reaching the darn thing.
Concerning external US:
If you are lucky and have a good ultrasound and ultrasonographer, and a small mare, then it might be a possibility, but consider that there is a lot of tissue between the ultrasound probe and the fetus...
Basically a blood test is about the most reliable to determine pregnancy.Well unless you can see or feel movement!
From what I have been told an internal US would not have seen the foal. Because she was in the window where the foal dropped over the pelvic brim. I say have the vet out to draw blood.