I have great admiration for all those sensible folks who pregnancy test/US/WeeFoal/milk test etc and get themselves so well organised. Me? I'm so very lacking in these things - lazy!! LOL!!
Decide when I want foals to be born, cover mares at the appropriate time, dont re-cover if I dont want later foals, and then just wait!! Luckily mine can munch grass all year round, so that together with special licks is all they need unless we have heavy snow and then they get ad lib big round bales put out for them. Couple of months from foaling time the mares that have been covered are usually moved to their own pasture where they are watched more closely for end of pregnancy signs. A month before foaling dates (or as soon as there is any udder development if this comes first) they are stabled at nights and a balancer feed is given - remember my girls are all overweight! - and nightly checks start. I know by this time whether they are in foal or not obviously, and as they progress the night time visits are increased until we are just sitting there waiting for the new baby to arrive.
So, as I said, it's the lazy way of doing things, playing the waiting game! If the mare is pregnant she will eventually produce her baby, nature is funny like that! Just read the signs, be ultra observant and dont just THINK she might foal tonight/tomorrow etc., BE THERE with her just in case!!
Sorry Tina, got a bit carried away as usual! I think you must do exactly what makes you happy, like Diane says, anyway I cannot think that any tests done would cause a problem - they wouldn't be allowed if they caused damage in any way would they?