I have had a couple of mares get in foal and the U/S show nothing...it happens all the time. I am very reluctant to allow vets to give my mares, especially older mares drugs to short cycle them because the fetus does not always show! I really do wonder how many they kill because they don't see them.
I had one mare that was huge before they could palpate and feel a foal, then she foaled about four weeks later. Modern technology sometimes leaves desire for improvement!
Billie, I would trust your experiences then, but have you had an ultrasound show a lot of fluid? That usually indicates infection, but can it sometimes be because of pregnancy? With all of the fluid I am guessing a fetus would be hard to pick up on, but she is already at 6 months and this was done at 4 months? I would think that was far enough along to see something.