As I stated earlier, most abortion diagnostic workups fail to determine a definitive cause of abortion - but they do 'rule out' many of the things that you, as a producer, can influence - vaccination, mineral supplementation, testing forages for excessive nitrates, etc.
If I was only allowed to have one specimen to work with... I want placenta. But... so often, placenta is not submitted - the cow ate it, the buzzards/coyotes/dogs ate it, it's still in the cow as she goes over the hill, or the producer and vet just don't consider sending it. In probably 50% of the cases where I receive placenta, there is a lesion or pathogen detected in placental tissues - and absolutely no abnormal findings in the fetus.