It certainly would be a great thing if we could ultrasound for these defects - but we cannot. There are a couple of reasons
Ultrasound gives you a slice of the fetus (ie like looking at one slice of bread in a loaf) in real time - liquid is black, bone is white, the fetus is moving - gross abnormalities like shistosoma refluxes (all guts out of abdomen, and other nasty things) can be picked up in early pregnancy - but once the fetus falls over the brim of the pelvis you are lucky to see much of anything.
Starting with TH - in the breeding studies of Galloway TH induced abortion of the fetus was able to identify TH by necropsy at 82 days - however these abnormalities are fairly subtle at this age and an ultrasound is incapable of distinguishing these subtle changes.
PHA - we don't exactly know when the fluid accumulates in the fetus in PHA - it appears to be after 5 months - when the fetus is over the brim of the pelvis. The subtle distinction of fluid in the fetus with fluid that surrounds it with a slice of fetus visible with the little bugger moving...can't be done.
I have actually asked both the bovine practitioners list and the reproductive ultrasound list and although we all think it would be a great idea it is not technically possible.
To your second question - a friend who had the misfortune to live and practice in an area where there were lost of PHA calves believes that at fall preg check (for spring born calves) you can palpate a difference in the PHA calves vs normals - ie there is more fluid in the uterus and the calf doesn't ballot like a normal calf.His clients think he is brilliant because he can tell - I suspect if you hadn't palpated a bunch of PHA calves you might be able to say something was fishy in Denmark, but not necessarily know what the issue is. There are several conditions that lead to increased uterine fluid in pregnant cows --- hope that helps...