I'm working with a veterinarian right now who has a seedstock producer client. Have had several small groups of cows that were confirmed pregnant on blood test at less than 60 days - back in March. Now have about a 20% 'open' rate in several of those groups. Of course, no one noticed those cows aborting - and depending upon WHEN they aborted, there may not have been much to see; unless you were right there when she dumped that small package of fetal membranes, you wouldn't necessarily know.
Folks - even veterinarians - talk about cows 'reabsorbing' pregnancies. That doesn't happen. They abort. It's just that if it's early in the pregnancy, you're rarely gonna find the fetus or placenta, except by sheer accident.
Thus far, all serologic testing for infectious abortifacient agents has come up negative, and we're now looking at mineral status(Cu, Se, Mn, Fe, etc.) - but this is a well-managed herd, and I don't anticipate finding anything out of whack...
I never got enough practice palpating to feel comfortable with confirming anything less than 45-60 days; colleagues who were in cows all day long every day...were probably consistently reliable at 28 days. But...some of those 28 day pregnancies are not gonna persist...
And, in my hands...anything beyond 4 months is just 'bred'. I might stick my neck out and say second or third trimester, but so far as saying 7 months, 8 months, or making a projection as to the size of the calf...I can't do it - and think that most people who do so are likely blowing smoke, for the most part.