Best shot at coming up with a definitive identification of cause of abortion requires examination of placenta, fetus, placenta, maternal serum samples (acute & convalescent),placenta...and did I say placenta? Even then, chances are well under 50% that a definitive cause will be identified.
I probably receive placenta in less than 10% of submissions - but in the majority of those, there is a lesion or pathogen identified in placental tissues - and NOTHING in the fetal tissues. When all I receive to examine is the fetus...I know from the outset that it's unlikely that I'm going to be able to tell that producer and their veterinarian why the fetus was aborted.
So...in most cases, I approach those abortion/stillbirth diagnostic cases with the hopes of 'ruling out' the common infectious causes that the producer and veterinarian can do anything about.
Historical info is also important...environment, hay/feed, water source, mineral supplementation program, vaccination history, fenceline contact with other cattle?, any recent additions to herd?, any animals going to shows and returning? any other animals aborting? - if so, when?, over how long a period of time?, heifers or mature cows affected?, etc.