If you don't have diagnostics done, you'll never know... hopefully this is not the first in a coming storm of aborted/stillborn calves.
That said, as a veterinary pathologist who's also a beef producer and former large-animal practitioner, I can tell you that abortion/stillbirth cases are potentially the most frustrating and least rewarding of any diagnostic pursuits. We can usually pretty well 'rule-out' the things that you and your veterinarian can do something about... IBR, BVD, Lepto, Neospora, etc. But, so far as actually being able to pinpoint exactly WHY a calf was aborted... probably less than 25% of the time.
Why is that, you might wonder? Here's some rambling thoughts...
A lot of things come into play...
1. decomposition/degeneration, whether in utero before expulsion, or afterwards, before the producer finds them...
2. scavenging by critters - I get 'em submitted more often than you'd think that are just an 'empty shell', with no internal organs left...
3. No placenta to examine. I know... the cow may have eaten it, the dogs/coyotes/buzzards may have eaten it, it may still be in the cow, as she goes over the top of the next hill... but, when I get placenta to examine - about 50% of the time, there's something going on in those fetal membranes - and NOTHING in the tissues from the calf itself! If placenta is not available or just not submitted for examination... the chance of getting a diagnosis is easily cut in half.
4. It's not an infectious or toxic problem. Could be a lethal genetic defect, etc. Back in the early 1990s, diagnosticians at the UofSaskatchewan diagnostic lab cultured fibroblasts(connective tissue cells) from the pericardial sacs of third-trimester abortions that were not decomposed at presentation... then karyotyped them; about 11% had chromosomal abnormalities that may or may not have been a lethal defect.
5. No paired maternal sera. To really do a proper job of 'blood-testing' for abortion agents, we really need a serum sample from the dam at the time of abortion, and a 'convalescent' sample collected 2-3 weeks later - so that we can look for increasing or decreasing antibody titers to the various agents. Sometimes we get the first sample... but rarely the just-as-important second one.
6. Disease in the cow... sometimes it's a cow issue... and the vets and diagnosticians aren't always privy to that info. Several years ago, a producer was losing some ET calves near term... we found NOTHING in the calves or placental membranes. But, when the cows started dying of anaplasmosis about 5 days later... Bingo!
Stillbirths/perinatal deaths probably skew the 'definitive diagnosis' numbers downward. These are full-term or near-term calves, either delivered dead or found dead soon after birth. So many things can go wrong... prolonged labor due to fetal/maternal size mismatch, abnormal pelvic structure in the dam, malpresentations, posterior deliveries, etc...that end with a dead, but completely normal calf. Some years, we see lots of perinatal deaths, particularly in cold winter weather, when dams have been on a protein-deficient diet... calves born to protein-deficient dams generate less body heat and take much longer to get up and nurse... and if it's really cold, they may just lie there and freeze to death.
