Sylvia, sheep
do abort just because they are startled. If they are early on and the egg is not yet firmly implanted, a scare (or bad weather, come to that) can cause implantation to fail.
We had one day second cycle when 19 of our ewes in four different groups all came a-tupping. That's nearly 10% of the total breeding ewes with tups at that time. Something had happened, whether it was weather or a rampaging dog through the farm, to cause that many ewes to lose their embryos.
They can also come down with pregnancy toxaemia later on in pregnancy without a great deal in the way of stress if they are carrying twins or triplets, for instance. It certainly doesn't take anything like as much as sustained chasing. And that's the point, I think - those of us with a lot of sheep / lambing experience know that it really doesn't have to be bad dogs behaving badly, it can be what to the owner - and dog - appears to be nothing much and yet it can threaten a pregnancy, the life of the lamb(s) and even that of the ewe.
Annie, no-one can blame someone walking past on the road with a dog on a lead. I'm not sure what I think about a dog being off lead on the road at other times of the year but in lambing country at lambing time, then from the farmer's perspective, it'd be better if the dog were on the lead, for sure. We move some of our couples from the lambing pens to the fields along the lanes, I would be pretty upset to run into dogs off lead en route - although so long as the dog is leashed and controlled by the time we get up to it I suppose there'd be no harm done.
It isn't always possible to have all of ones lambing ewes in fields nowhere near roads or footpaths - or now of course, also 'open access land'.
In fact, what with the constraints of environmental schemes, two roads, the Roman Fort, Hadrian's Wall, two public footpaths and quite a few well-used but informal local routes, it is only about one-third of our fields (and considerably less than one-third of the acreage) in which we are allowed to graze sheep and which have neither footpath nor significant road frontage.
And we have to try to keep cattle off the tourist routes, especially cows with calves, in case of 'incident' - so our choices can be somewhat limited for safe, quiet places to lamb!
So what we do is try to make sure that the ewes who will be pregnant and lambing in those fields are used to walkers and other traffic; for the most part we keep our girls in the same fields from tupping to lambing, so they should be inured to the regular types of passers-by by the time the lambs are imminent. But it does mean that many of them are in fields that people will bring dogs through, so we just have to keep our fingers crossed that all the dogs and owners behave sensibly - and that the ewes do, too!