Agree with redcowsrule - that's the ideal specimen for an abortion diagnostic workup - FRESH fetus and placenta... and from personal experience as a veterinary pathologist, the placenta is often the most important tissue to examine - but it's often not available (eaten by cow/dog/buzzard... or retained in the cow).
Be aware, however that we (laboratory diagnosticians) rarely determine a definitive cause of abortion in more than about 25% of cases, but not all abortions are infectious in nature, and once we rule out the 'important' pathogens (Brucella, Leptospira, Neospora,IBR, BVD, Anaplasmosis/Babesiosis/Theileriosis, etc.) and toxins like Nitrate... it's a low-yield endeavor. So... I tend to look at Abortion diagnostic examinations as... "We're going to rule-out the potential causes that you as a producer &/or veterinarian can do anything about - with vaccination, nutrition, management, etc. "