What Ken said... abortions often come accompanied by retained placenta.
If you give it a gentle tug and it comes out... good. But if it seems still firmly attached, it needs additional time to 'uncouple'
The majority of cows with retained placenta have no systemic illness - they still have good appetites, and may seem totally unconcerned by the god-awful stinking mess hanging out their back end. Unless the cow is off feed or noticeably ill, there's no need for antibiotics.
Will an occasional cow get sick/septic? Yeah, but they are definitely in the minority.
I've had cows with dead calves inside them that have turned into stinking, chunky 'soup', that you could just scoop out by the armful, bones coming apart, etc. - and the cow standing there peacefully chewing her cud.
That 6-month fetus will be about the size of a cat. Easy to miss, especially in a 'wooded' pasture.
Abortion/stillbirth diagnostic workups are often unrewarding - other than 'ruling out' the things that you can actually do something about - like vaccinating against lepto, IBR, BVD. Playing the odds, I would bet a Coke that Neospora is the likely cause of abortion in this case.
afs.ca.uky.edu