Early in the pregnancy, the vet actually gets a feel for what ours called the "slip"... has to do with how much fluid is in the uterus and the horn.... if real early, like 25 to 35 or 45 days, if the vet isn't very light with the touch, they can often slip the pregnancy...
Once it goes over the pelvis and into the body cavity, when it is in the 4+ month range, then it is somewhat on size of fetus and also on size of the cotolydens that attach the placenta to the walls of the uterus...
And a cow very close to calving can sometimes feel like they are not that close... or even "empty" because the calf is so far down in the "belly" that they can't always reach it well... so you think it is farther off...
And I am no "hand" at feeling the slip when shorter bred... I can tell after about 5 months and then it is a "definitely pregnant" or I don't feel anything...but not 100 % sure....