If you want to do diagnostics, you could either call your vet to submit samples, or do it yourself.
I worked in the virology lab at Wyoming State Vet Lab while in college, and we tested aborted fetuses for BVD, IBR, and Chlamydia (if I remember correctly!).
If interested in testing, just put in separate zip lock bags a piece of lung, liver, kidney, heart and spleen from the fetus. I'd submit enough tissue to fill half of a sandwich-sized ziploc bag. Also, if you could submit a chunk of afterbirth w/ cotyledons (big red buttons where placenta was attached to uterus) in a bag, that would be helpful too. Refrigerate until you ship. Likely the lab will do additional testing from other labs (bacteriology...) to try to diagnose the issue.
Be sure to send w/ ice packs and your contact info., as well as info. that specimens were from a near term, aborted bovine fetus. Also include vaccination history of dam.
You may want to verify this advice w/ your vet first, (I'm definately not a vet!), but I think the protocol should be similar. Also, you may want to call the lab before submitting samples to check prices (or look online).
It is frustrating and nerve-racking to lose them right before calving. Hang in there and Good Luck!