I'm not one to recommend indiscriminate antibiotic use, but if it were me, I'd be dosing every ewe on the place with LA200 or similar tetracycline, to head off an abortion storm. Those lambs are too early, and I'd be concerned about an infectious cause. Tetracycline is a good, cheap, and effective way to head off many of the common causes of such a storm, and injecting it is the fastest, surest way to get it in them. Talk to your vet if you need to, but do it quick would be my advice.
Phil Crome