I know it's just an old wives tale, so often doesn't work, but the string test is still kind of fun.
Anyway, I tested all my mares over the winter. The two that were never exposed, the nail never moved. The other 4 it moved. Today, I tested two mares that already foaled, no movement; the exposed mare that hasn't foaled the nail moved. So we'll see what happens, but we are getting towards the end of decent gestation time frame, so hard to say what may happen.