Found this on mucous plug
Blood-spotting in itself isn't necessarily cause for alarm - it's always much more impressive-looking against a grey coat, remember. It's not unusual, and may be quite normal. Sometime before foaling, usually a week to ten days before foaling, sometimes just a few days before, a mare will lose the mucous plug that has been blocking her cervix. If the mare is dark and you're not looking under her tail, you may not notice this at all; if the mare is light and you ARE looking, this can look like a small, slightly bloody discharge. The plug can come out all at once, or slowly and over a few days. If you've been noticing a significant discharge for more than a few days in a row, or if there's a good deal more blood than would typically be associated with a mucous plug (just enough to turn it pinkish-brown), then it would be a good idea to have the vet take another look. Don't panic - it's not necessarily anything horrible or dangerous. It's possible that she may have a ruptured vaginal blood vessel, which, if it's a small one, isn't necessarily cause for alarm. And although it's not typical, some mares lose that mucous plug two weeks or more before foaling - and some mares show a little blood every day for a week or two before producing a perfectly normal foal.
